feat: Support RESET ALL command

[apstndb]

Implement RESET ALL command to reset all session variables to their default values.

Reference:

• java-spanner: Supports RESET ALL as a client-side statement that resets all properties to defaults within the current transaction scope.
• go-sql-spanner: RESET command exists in the parser for individual properties.

Implementation notes:

• Add RESET ALL to client_side_statement_def.go
• Iterate over all resettable system variables and restore defaults
• Consider interaction with transaction-scoped variables (feat: Support SET LOCAL for transaction-scoped variables #400)