feat: add AWS-compatible standalone IAM service

Closes #1640

Add a standalone AWS IAM Query API implementation for managing IAM users through standard AWS SDKs and the AWS CLI.

Server usage

Start the IAM server with internal file-backed storage:

    mkdir -p /tmp/versitygw-iam
    ./versitygw --port 127.0.0.1:7070 --access user --secret pass iam --dir /tmp/versitygw-iam

Start the IAM server with Vault KV v2 storage using AppRole:

    VGW_IAM_VAULT_ROLE_SECRET=<role-secret> ./versitygw --port 127.0.0.1:7070 --access user --secret pass iam --vault-endpoint-url http://127.0.0.1:8200 --vault-auth-method approle --vault-role-id <role-id> --vault-mount-path kv --vault-secret-storage-path iam

Vault authentication also supports root tokens, separate authentication and secret-storage namespaces, custom mount paths, server certificate validation, and mutual TLS client certificates.

Configure the AWS CLI credentials used by the IAM server:

    export AWS_ACCESS_KEY_ID=user
    export AWS_SECRET_ACCESS_KEY=pass
    export AWS_DEFAULT_REGION=us-east-1

Implemented IAM actions

CreateUser creates an IAM user with an AWS-compatible ARN, generated AIDA user ID, creation timestamp, optional path, and tags. It validates usernames, paths, tag limits, reserved tag prefixes, duplicate tag keys, and existing users.

    aws --endpoint-url http://127.0.0.1:7070 iam create-user --user-name bob

    aws --endpoint-url http://127.0.0.1:7070 iam create-user --user-name bob --path /engineering/ --tags Key=team,Value=storage

GetUser returns a stored user or the root identity when requested without a username through the IAM Query API.

    aws --endpoint-url http://127.0.0.1:7070 iam get-user --user-name bob

ListUsers returns users in deterministic username order and supports path filtering, marker-based pagination, and MaxItems limits.

    aws --endpoint-url http://127.0.0.1:7070 iam list-users

    aws --endpoint-url http://127.0.0.1:7070 iam list-users --path-prefix /engineering/ --max-items 100

UpdateUser updates the username and/or path, recalculates the user ARN, and rejects conflicts with existing users.

    aws --endpoint-url http://127.0.0.1:7070 iam update-user --user-name bob --new-user-name robert --new-path /platform/

DeleteUser permanently removes an IAM user and returns AWS-compatible errors for missing users.

    aws --endpoint-url http://127.0.0.1:7070 iam delete-user --user-name robert

IAM protocol and authentication

- Support the AWS IAM Query protocol version 2010-05-08 over GET and POST form requests.
- Return AWS-compatible XML responses, error documents, status codes, request IDs, user metadata, and pagination fields.
- Authenticate root credentials with AWS Signature Version 4 for the IAM service in us-east-1.
- Support both Authorization-header and query-string SigV4 authentication.
- Validate credential scope, signed headers, timestamps, clock skew, content length, signatures, and unsupported signature or session-token modes.
- Add IAM-specific validation and error mapping for malformed requests, invalid actions, duplicate entities, missing users, throttling, and internal failures.

Storage implementations

- Add an internal JSON-backed store using iam.json and iam.json.backup with atomic temporary-file replacement, concurrent access protection, stable ordering, pagination, and persistence across restarts.
- Add a Vault KV v2 store with one secret per user, CAS-based duplicate protection, permanent deletion, AppRole reauthentication, namespace support, configurable authentication and KV mounts, root-token authentication, and TLS/mTLS configuration.
- Introduce a common Storer interface and require exactly one storage backend to be configured.

Server and embedding support

- Register the new `versitygw iam` command with environment-variable and CLI configuration for both storage backends.
- Add `embedgw.RunIAMAPI` and `IAMConfig` for embedding the IAM service in Go applications.

Gateway-level internal packages

- Add `internal/iamstore` as a reusable generic file-backed IAM persistence engine and migrate the existing gateway internal IAM service to it.
- Add `internal/sigv4auth` for shared SigV4 header and presigned-query parsing, canonical request generation, signature verification, and structured authentication errors.
- Refactor the S3 authentication paths to use the shared SigV4 implementation while preserving S3-specific error responses.
- Add `internal/httpctx` for shared Fiber context keys and AWS-style request ID handling.
- Add `internal/routekit` for shared query, form, and header route matchers.
- Add `internal/netutil` for reusable certificate storage, hostname-aware listeners, multi-address serving, TLS listeners, and UNIX socket handling.
- Update the custom SigV4 signer to honor an explicitly supplied signed-header list so unrelated headers do not alter IAM signatures.

Testing and CI

- Add AWS IAM SDK-based integration coverage for all supported user actions, header authentication, query authentication, validation, errors, filtering, and pagination.
- Split standalone IAM tests into `versitygw test iam` and retain existing gateway IAM tests under `versitygw test gw-iam`.
- Add unit coverage for controllers, authentication, routing, storage, embedding, listeners, request matching, persistence, and signing behavior.
- Add `runiamtests.sh` to exercise internal storage over HTTP and HTTPS plus Vault storage through AppRole.
- Add a dedicated IAM functional-test workflow with a Vault service and merged runtime coverage reporting.
- Include the IAM test runner in shellcheck and add the AWS IAM SDK dependency.

s3api/utils/utils.go

@@ -170,57 +170,6 @@ func createHttpRequestFromCtx(ctx fiber.Ctx, signedHdrs []string, contentLength
 	return httpReq, nil
 }
 
-var (
-	signedQueryArgs = map[string]bool{
-		"X-Amz-Algorithm":     true,
-		"X-Amz-Credential":    true,
-		"X-Amz-Date":          true,
-		"X-Amz-SignedHeaders": true,
-		"X-Amz-Signature":     true,
-	}
-)
-
-func createPresignedHttpRequestFromCtx(ctx fiber.Ctx, signedHdrs []string, contentLength int64) (*http.Request, error) {
-	req := ctx.Request()
-
-	uri, _, _ := strings.Cut(ctx.OriginalURL(), "?")
-	isFirst := true
-
-	for key, value := range ctx.Request().URI().QueryArgs().All() {
-		_, ok := signedQueryArgs[string(key)]
-		if !ok {
-			escapeValue := url.QueryEscape(string(value))
-			if isFirst {
-				uri += fmt.Sprintf("?%s=%s", key, escapeValue)
-				isFirst = false
-			} else {
-				uri += fmt.Sprintf("&%s=%s", key, escapeValue)
-			}
-		}
-	}
-
-	httpReq, err := http.NewRequest(string(req.Header.Method()), uri, nil)
-	if err != nil {
-		return nil, errors.New("error in creating an http request")
-	}
-	if err := addRequestHeadersFromCtx(ctx, httpReq, signedHdrs); err != nil {
-		return nil, err
-	}
-
-	// Check if Content-Length in signed headers
-	// If content length is non 0, then the header will be included
-	if !includeHeader("Content-Length", signedHdrs) {
-		httpReq.ContentLength = 0
-	} else {
-		httpReq.ContentLength = contentLength
-	}
-
-	// Set the Host header
-	httpReq.Host = string(req.Header.Host())
-
-	return httpReq, nil
-}
-
 func SetMetaHeaders(ctx fiber.Ctx, meta map[string]string) {
 	ctx.Response().Header.DisableNormalizing()
 	for key, val := range meta {