[Improvement] make gcs-service-account-file optional in fileset catalog configuration

### What would you like to be improved?

If Gravitino run on Google cloud storage, no need to specify the credential path.

### How should we improve?

_No response_

Inspect the fileset catalog configuration and the Google Cloud Storage credential handling; the issue does not name specific files or tests. Determine where gcs-service-account-file is required, then verify that GCS works without a credential path while preserving explicit credential-path behavior.

Written by the indexing model from the issue text.