Periodic authorization expiration checks using GoAway message

This patch introduces a mechanism to address the security issue of stale,
authorized connections persisting beyond their validity period. .
Currently, once a session is established, postgres does not automatically
re-validate credentials. If a password expires (rolvaliduntil) the session
remains active indefinitely. Same applies to centralized authentication
systems (like Kerberos or OAuth).

This patch depends on the "GoAway" protocol message proposal currently
under review here:
https://www.postgresql.org/message-id/DDPQ1RV5FE9U.I2WW34NGRD8Z%40jeltef.nl
Please apply this patch on top of the GoAway patch.

The Solution: To handle this authorization gap gracefully, this patch
leverages the pending GoAway protocol message to notify clients.

Please find below summary of the solution

New GUC: auth_expiration_check_interval (integer, minutes). Controls the
frequency of checking a session's authorization status. Setting it to 0
(default) disables the check.

Periodic Idle Check: When a backend process is idle (waiting for the next
command) and the timeout is reached, the server calls a placeholder
function check_external_auth_status_expired().

Graceful Disconnect: If authorization is revoked/expired, the server sends
the GoAway message. This allows the client to finish any current
processing and reconnect cleanly.

Thanks & Best Regards,
Ajit Awekar