Added check for wal_retrieve_retry_interval parameter ALTER SYSTEM privilege presence

[RealGreenDragon]

The Repmgr standby_disconnect_on_failover option requires to edit permanently (ALTER SYSTEM privilege) wal_retrieve_retry_interval system parameter, privilege reserved to superusers only until PostgreSQL 14.

From PostgreSQL 15 it is changed, as reported on v15 changelog:

Allow GRANT to grant permissions to change individual server variables via SET and ALTER SYSTEM (Mark Dilger)

The new function has_parameter_privilege() reports on this privilege.

This PR add the check of wal_retrieve_retry_interval parameter change privilege via ALTER SYSTEM if repmgr database user is not a superuser.

[RealGreenDragon]

Fixed PR title and description as the privilege name is parameter, instead ALTER SYSTEM is the particular parameter privilege to change the value permanently and not only in current session (SET privilege).

[RealGreenDragon]

Any news on this PR?