fix: LoginSTS should be an inline implementation (#11337)

STS tokens can be obtained by using local APIs
once the remote JWT token is presented, current
code was not validating the incoming token in the
first place and was incorrectly making a network
operation using that token.

For the most part this always works without issues,
but under adversarial scenarios it exposes client
to hand-craft a request that can reach internal
services without authentication.

This kind of proxying should be avoided before
validating the incoming token.

cmd/api-errors.go

@@ -2125,6 +2125,12 @@ func toAPIError(ctx context.Context, err error) APIError {
 				HTTPStatusCode: e.Response().StatusCode,
 			}
 			// Add more Gateway SDKs here if any in future.
+		default:
+			apiErr = APIError{
+				Code:           apiErr.Code,
+				Description:    fmt.Sprintf("%s: cause(%v)", apiErr.Description, err),
+				HTTPStatusCode: apiErr.HTTPStatusCode,
+			}
 		}
 	}