feat(transaction): add ExpireSnapshotsAction for snapshot lifecycle management

[drbothen]

Which issue does this PR close?

• Closes Add ExpireSnapshotsAction for snapshot lifecycle management #2145
• Ref Maintenance: Expire Snapshots Action #1454, feat(core): expose remove_snapshots at Transaction API #884

What changes are included in this PR?

Adds ExpireSnapshotsAction to the transaction API, enabling removal of old snapshots from table metadata. Key features:

• Explicit expiry: Expire specific snapshot IDs via expire_snapshot_ids()
• Age-based expiry: Expire snapshots older than a timestamp via expire_older_than_ms()
• Safety guarantees: Current snapshot is never expired; retain_last(n) ensures a minimum number of snapshots survive
• File cleanup utility: find_unreferenced_files() identifies data files, manifests, and manifest lists that are only referenced by expired snapshots and can be safely deleted after commit

The action returns a RemoveSnapshots table update and does NOT delete physical files directly, following the Iceberg principle of metadata-first operations.

Are these changes tested?

Yes — 5 unit tests covering:

• Current snapshot protection
• Age-based expiry
• retain_last enforcement
• No-op when nothing to expire
• find_unreferenced_files with carried-forward files