Improve monitoring of Cross-Site-Replication

Cross site Replication should be monitored on a postgresql level. Today we discovered an issue with cross-site replication not working. The secondary site didn't replicate. But there were no alerts or anything pointing us to it. I only found it because I was testing a new user for a database.
We need a way to continuously monitor cross-site replication and throw a warning via cos if it stops working.

This is adjacent to #1808, but in this case juju did work (even switchovers on the primary were correctly communicated to the secondary via relation), but the postgresql replication didn't work.

No files or tests are named in the issue. Start by examining how PostgreSQL cross-site replication status can be checked and where COS warnings are emitted; done means a stopped replication link is detected continuously and produces a warning, including when Juju relations and switchovers still work.

Written by the indexing model from the issue text.