c2c: ping primary (source) cluster from the standby (destination) cluster

**Is your feature request related to a problem? Please describe.**

When operators intend to put Physical Replication into production, we can anticipate that they will need to troubleshoot the connectivity between destination and source.

Problems might include routing, DNS, certificate authorities.

**Describe the solution you'd like**

A command in the spirit of [`SHOW BACKUP CONNECTION`](https://github.com/cockroachdb/cockroach/pull/97517). That command pings cloud storage (such as S3) from all nodes in a cluster, and reports back.

Let’s extend it to support `postgres://` URLs. Or, as suggested by @msbutler, extend `SHOW TENANT` with a flag.

(Maybe the semantics should be pinging between _tenants_, specifically? Not sure.)

**Describe alternatives you've considered**

Users will see errors when they attempt to start replicate via `CREATE TENANT application FROM REPLICATION OF application ON 'postgresql://the connection’`. Maybe that’s enough?

Jira issue: CRDB-29512