feat: add IAM OIDC provider tagging actions

Adds `TagOpenIDConnectProvider`, `UntagOpenIDConnectProvider` and `ListOpenIDConnectProviderTags` to the standalone IAM service, backed by both the internal and Vault storers. They follow the user and role tagging actions in most respects — the tag action merges into the provider's existing tags and rejects a repeated key, untag removal is idempotent, and the tag listing is sorted by key and paginated, with the per-request member count and the per-provider tag total enforced as separate quotas so replacing a tag on a provider already at the 50-tag cap still succeeds — but differ in the one respect IAM itself draws: OIDC provider tag keys are compared exactly, not case-insensitively. On a provider `env` and `ENV` are two independent tags, both may be supplied in a single request, only a byte-identical repeat is a duplicate (reported without the "Tag keys are case insensitive" note the user and role actions carry), and untagging `env` leaves `ENV` in place.

That distinction is now carried by `iamutil.TagKeyCase`, which `ParseTags` uses for duplicate detection and which `mergeTags`, `removeTags` and the tag listing's marker lookup use for key matching. `CreateOpenIDConnectProvider` moves onto the exact comparison too, so a provider created with case-differing tag keys keeps both.

All three actions are authorized against the target provider's ARN, so `aws:ResourceTag/<key>` reads the provider's own tags, and the tag and untag actions populate `aws:RequestTag/<key>` and `aws:TagKeys` respectively, so a tag-scoped policy Condition governs which tags a caller may set or remove. All three report a missing provider with the wording `DeleteOpenIDConnectProvider` uses rather than the one `GetOpenIDConnectProvider` uses, which is why the Vault provider read now takes the not-found error its calling action reports.

The WebGUI gains a Tags section in the OIDC provider manage view, replacing the read-only tag row, and the shared tag editor gains a case-sensitive mode that changes its duplicate-key check, its diffing of an edited set into an untag and tag pair, and the wording of its guidance.

iamapi/iamerr/errors.go

@@ -62,6 +62,7 @@ const (
 	ErrTagLimitExceeded
 	ErrInvalidPathPrefix
 	ErrDuplicateTagKeys
+	ErrDuplicateExactTagKeys
 	ErrInvalidAccessKeyIDChars
 	ErrDeleteConflict
 	ErrDeleteConflictPolicies
@@ -262,6 +263,12 @@ var errorCodeResponse = map[ErrorCode]Error{
 		Message:        "Duplicate tag keys found. Please note that Tag keys are case insensitive.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrDuplicateExactTagKeys: {
+		Type:           TypeSender,
+		Code:           "InvalidInput",
+		Message:        "Duplicate tag keys found.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrInvalidAccessKeyIDChars: {
 		Type:           TypeSender,
 		Code:           "ValidationError",
@@ -540,6 +547,9 @@ func NoSuchEntityOIDCProviderGet(arn string) Error {
 	return newSenderError("NoSuchEntity", fmt.Sprintf("OpenIDConnect Provider not found for arn %s", arn), http.StatusNotFound)
 }
 
+// NoSuchEntityOIDCProviderDelete is the wording DeleteOpenIDConnectProvider
+// and the provider tagging actions use, distinct from the one
+// NoSuchEntityOIDCProviderGet reports.
 func NoSuchEntityOIDCProviderDelete(arn string) Error {
 	return newSenderError("NoSuchEntity", fmt.Sprintf("OpenId connect Provider %s cannot be found.", arn), http.StatusNotFound)
 }