Add ExpireSnapshotsAction for snapshot lifecycle management

[drbothen]

Is your feature request related to a problem or challenge?

There is currently no way to remove old snapshots from table metadata via the transaction API. Over time, tables accumulate snapshots which increases metadata size and makes operations slower. Issue #1454 was filed for this but closed without an implementation landing.

Describe the solution you'd like

Add an ExpireSnapshotsAction that:

1. Removes specified snapshots from table metadata (by ID or by age cutoff)
2. Respects a minimum retention count (always keeps at least N most recent snapshots)
3. Never expires the current snapshot
4. Returns RemoveSnapshots table update (does NOT delete physical files)

Also add a find_unreferenced_files() utility that identifies data files, manifests, and manifest lists that are only referenced by expired snapshots and can be safely deleted.

Willingness to contribute

I am willing to submit a PR for this feature.