feat: Support TRANSACTION_TIMEOUT system variable

[apstndb]

Implement TRANSACTION_TIMEOUT system variable for controlling transaction-level timeouts.

Reference:

• java-spanner: Supports TRANSACTION_TIMEOUT as read/write [LOCAL] variable with duration or NULL values.
• go-sql-spanner: Supports transaction_timeout as a connection property (time.Duration).

Implementation notes:

• Add TRANSACTION_TIMEOUT to system variable registry
• Apply timeout to read/write transactions
• Support [LOCAL] scoping when feat: Support SET LOCAL for transaction-scoped variables #400 is implemented