Support `retainLast` and `setMinSnapshotsToKeep` Snapshot Retention Policies

[ForeverAngry]

Feature Request / Improvement

Overview

The Apache Iceberg Java API provides more robust and flexible retention options for expiring Snapshots, including:

• retainLast(n) – Retain the most recent n snapshots.
• setMinSnapshotsToKeep(n) – Ensure a minimum number of snapshots are always retained.

Description

Adding support for non-time-based retention strategies provides better control over snapshot pruning. This ensures operational resilience while still allowing space reclamation. For example:

• retainLast(n) is helpful when regular snapshot creation occurs and users always want to keep the last few for rollback.
• setMinSnapshotsToKeep(n) acts as a guardrail to prevent aggressive expiration logic from removing too many snapshots.

[ForeverAngry]

@greenlaw take a look, let me know if missed anything you were looking for, i added the enhancements outlined above to: #2143