feat: Support AUTO_BATCH_DML_UPDATE_COUNT and its verification

[apstndb]

This feature is about providing finer control over auto-batched DML statements.

Reference (java-spanner & go-sql-spanner):
These libraries support AUTO_BATCH_DML_UPDATE_COUNT and AUTO_BATCH_DML_UPDATE_COUNT_VERIFICATION as system variables. This allows users to specify the expected update count for buffered DML and to verify it against the actual count returned by Spanner.

Implementation suggestion for spanner-mycli:

• Add AUTO_BATCH_DML_UPDATE_COUNT and AUTO_BATCH_DML_UPDATE_COUNT_VERIFICATION to the system variable map.
• Implement the read/write logic for these variables, affecting the behavior of DML statements when AUTO_BATCH_DML is enabled.