feat: Support SAVEPOINT, RELEASE SAVEPOINT, and ROLLBACK TO SAVEPOINT

[apstndb]

This feature is about implementing transaction savepoints.

Reference (java-spanner):
java-spanner supports the standard SQL commands SAVEPOINT <name>, RELEASE SAVEPOINT <name>, and ROLLBACK TO SAVEPOINT <name>. This allows for fine-grained control over transaction rollbacks.

Implementation suggestion for spanner-mycli:
This would require significant changes to the internal transaction management logic to track and manage multiple rollback points within a single transaction.