feature request: new 'max-sigterm-delay' flag

The `--max-sigterm-delay` flag is crucial for microservices that require a graceful shutdown period to ensure ongoing transactions are completed successfully. This flag allows the microservice to remain operational for a specified duration after receiving a termination signal (SIGTERM), granting sufficient time for its database connector to remain active and handle pending transactions.

flag | description
--- | ---
--max-sigterm-delay | Maximum number of seconds to wait for connections to close after receiving a TERM signal.

Disclaimer: The name of the flag and the description are copied from the https://github.com/GoogleCloudPlatform/cloud-sql-proxy

Implementing this feature will smoothen the transition from Postgres to Spanner.