[Improvement] Clarify the `AuditInfo` value in Gravitino

### What would you like to be improved?

The current `AuditInfo` is mainly based on the information of the underlying catalog (if any), but Gravitino has started to build its own user system.

Should we still reflect the user information of external systems in the `AuditInfo`? AFAIK, both Unity Catalog and Starburst only use their own user systems.

### How should we improve?

Clarify the `AuditInfo` value in Gravitino

Start by locating the current AuditInfo definition and tracing how it derives information from an underlying catalog. Compare that behavior with Gravitino's own user system and the Unity Catalog and Starburst examples mentioned in the issue. Done means the intended source of user information and the resulting AuditInfo semantics are explicitly decided and documented.

Written by the indexing model from the issue text.