STS: Fallback to Caller Identity when RoleArn is missing in AssumeRole (#8345)

* s3api: make RoleArn optional in AssumeRole

* s3api: address PR feedback for optional RoleArn

* iam: add configurable default role for AssumeRole

* S3 STS: Use caller identity when RoleArn is missing

- Fallback to PrincipalArn/Context in AssumeRole if RoleArn is empty

- Handle User ARNs in prepareSTSCredentials

- Fix PrincipalArn generation for env var credentials

* Test: Add unit test for AssumeRole caller identity fallback

* fix(s3api): propagate admin permissions to assumed role session when using caller identity fallback

* STS: Fix is_admin propagation and optimize IAM policy evaluation for assumed roles

- Restore is_admin propagation via JWT req_ctx
- Optimize IsActionAllowed to skip role lookups for admin sessions
- Ensure session policies are still applied for downscoping
- Remove debug logging
- Fix syntax errors in cleanup

* fix(iam): resolve STS policy bypass for admin sessions

- Fixed IsActionAllowed in iam_manager.go to correctly identify and validate internal STS tokens, ensuring session policies are enforced.
- Refactored VerifyActionPermission in auth_credentials.go to properly handle session tokens and avoid legacy authorization short-circuits.
- Added debug logging for better tracing of policy evaluation and session validation.

weed/s3api/auth_signature_v4.go

@@ -434,6 +434,11 @@ func (iam *IdentityAccessManagement) validateSTSSessionToken(r *http.Request, se
 		Claims:       claims,               // Populate Claims for policy variable substitution
 	}
 
+	// Restore admin privileges if the session was created by an admin
+	// if isAdmin, ok := claims["is_admin"].(bool); ok && isAdmin {
+	// 	identity.Actions = append(identity.Actions, s3_constants.ACTION_ADMIN)
+	// }
+
 	glog.V(2).Infof("Successfully validated STS session token for principal: %s, assumed role user: %s",
 		sessionInfo.Principal, sessionInfo.AssumedRoleUser)
 	return identity, cred, s3err.ErrNone