Expose the last global transaction identifier (GTID) executed for a CONSISTENT SNAPSHOT.

STWCS stashes and provides to the session the binlog file and offset in variables snapshot_file and snapshot_position, but, it does not expose last_gtid_executed. This can be extracted manually from binlogs given the file name and pos, but is not terribly user friendly. So, this new feature discussion is about adding a new snapshot_last_gtid_executed that will capture the same at the beginning of the snapshot and allow users/scripts/tools to have easy access.