server: enhance _status/NetworkConnectivity for external-process mode

This is a follow-up of the PR https://github.com/cockroachdb/cockroach/pull/131830. The PR description is self-explanatory, but for completeness, I'll summarize it here.

With https://github.com/cockroachdb/cockroach/pull/131830, `_status/NetworkConnectivity` is now enabled for secondary tenants in both shared and external process modes. This works well for shared-process mode where inter-node network connectivity for a secondary tenant is the same as for the system tenant. However, in external-process mode, this endpoint won't provide a complete picture of network connectivity since the SQL server may run entirely outside the KV node. As a result, this endpoint only shows KV-KV nodes network connectivity so it isn't ready for external-process mode. We may need to extend this or create a new endpoint for SQL-SQL servers and SQL server to KV nodes.

Jira issue: CRDB-45934