Add SQL comment to streaming replication user

[gbartolini]

CloudNativePG creates a user for streaming replication matters, by default called "streaming_replica". It would be neat to make sure that, when we create the user, we add:

COMMENT ON ROLE streaming_replica IS 'Special user for streaming replication created by CloudNativePG';

We could think about enforcing this as comment at any time, but I have contrasting feelings. I think it should be fine to set comments at the start and let users manage them after.