[Improvement] Implementation of TTL for FileSets and Subdirectories in Gravitino

### What would you like to be improved?

What specific implementations are being considered by Gravitino for setting TTL (Time to Live) for FileSets and their subdirectories?

### How should we improve?

_No response_

The issue names FileSets and their subdirectories but provides no files, tests, or implementation entry points. Start by locating the existing FileSet and subdirectory handling, then clarify the intended TTL semantics and scope; done would require an agreed implementation plan or feature that defines and applies TTL behavior.

Written by the indexing model from the issue text.