Add ExpireSnapshots as a concrete implementation in iceberg/update/
following the PendingUpdate pattern.

Features:
- Fluent API builder methods: ExpireSnapshotId, ExpireOlderThan,
  RetainLast, DeleteWith, SetCleanupLevel
- Extends PendingUpdateTyped<vector<shared_ptr<Snapshot>>>
- Takes Table* in constructor for access to table metadata
- Placeholder implementations for Apply() and Commit()

Directory structure:
- src/iceberg/update/expire_snapshots.{h,cc}
- src/iceberg/update/CMakeLists.txt
- src/iceberg/update/meson.build
- Updated build files (CMakeLists.txt, meson.build)

Change Table::NewExpireSnapshots() and Transaction::NewExpireSnapshots()
to return std::unique_ptr instead of std::shared_ptr for consistency
with other factory methods (NewScan, NewTransaction) that create new
objects with exclusive ownership transfer to the caller.

Add PendingUpdateTyped template class to provide type-safe ApplyTyped() method
for table metadata changes. This allows different operations to return
specific result types (e.g., vector<Snapshot> for ExpireSnapshots).

Changes:
- Add PendingUpdateTyped<T> template extending PendingUpdate
- Add ApplyTyped() method returning Result<T> for typed operations
- Keep Apply() returning Status in base PendingUpdate for compatibility
- PendingUpdateTyped provides default Apply() implementation that calls ApplyTyped()
- Update ExpireSnapshots to use ApplyTyped() instead of Apply()

This design allows:
- ExpireSnapshots to return Result<vector<Snapshot>> from ApplyTyped()
- UpdateProperties to continue using Status Apply() from base
- All PendingUpdate subclasses to work polymorphically through base interface