feat(sts): pass filerAddress at call-time instead of init-time

This change addresses the requirement that filer addresses should be
passed when methods are called, not during initialization, to support:
- Dynamic filer failover and load balancing
- Runtime changes to filer topology
- Environment-agnostic configuration files

### Changes Made:

#### SessionStore Interface & Implementations:
- Updated SessionStore interface to accept filerAddress parameter in all methods
- Modified FilerSessionStore to remove filerAddress field from struct
- Updated MemorySessionStore to accept filerAddress (ignored) for interface consistency
- All methods now take: (ctx, filerAddress, sessionId, ...) parameters

#### STS Service Methods:
- Updated all public STS methods to accept filerAddress parameter:
  - AssumeRoleWithWebIdentity(ctx, filerAddress, request)
  - AssumeRoleWithCredentials(ctx, filerAddress, request)
  - ValidateSessionToken(ctx, filerAddress, sessionToken)
  - RevokeSession(ctx, filerAddress, sessionToken)
  - ExpireSessionForTesting(ctx, filerAddress, sessionToken)

#### Configuration Cleanup:
- Removed filerAddress from all configuration files (iam_config_distributed.json)
- Configuration now only contains basePath and other store-specific settings
- Makes configs environment-agnostic (dev/staging/prod compatible)

#### Test Updates:
- Updated all test files to pass testFilerAddress parameter
- Tests use dummy filerAddress ('localhost:8888') for consistency
- Maintains test functionality while validating new interface

### Benefits:
- ✅ Filer addresses determined at runtime by caller (S3 API server)
- ✅ Supports filer failover without service restart
- ✅ Configuration files work across environments
- ✅ Follows SeaweedFS patterns used elsewhere in codebase
- ✅ Load balancer friendly - no filer affinity required
- ✅ Horizontal scaling compatible

### Breaking Change:
This is a breaking change for any code calling STS service methods.
Callers must now pass filerAddress as the second parameter.

test/s3/iam/iam_config_distributed.json

@@ -6,7 +6,6 @@
     "signingKey": "dGVzdC1zaWduaW5nLWtleS0zMi1jaGFyYWN0ZXJzLWxvbmc=",
     "sessionStoreType": "filer",
     "sessionStoreConfig": {
-      "filerAddress": "localhost:8888",
       "basePath": "/etc/iam/sessions"
     },
     "providers": [
@@ -41,14 +40,12 @@
     "defaultEffect": "Deny",
     "storeType": "filer",
     "storeConfig": {
-      "filerAddress": "localhost:8888",
       "basePath": "/etc/iam/policies"
     }
   },
   "roleStore": {
     "storeType": "filer",
     "storeConfig": {
-      "filerAddress": "localhost:8888",
       "basePath": "/etc/iam/roles"
     }
   },