server: the "connectivity" status API endpoint should work with secondary tenants with sufficient capability

[knz]

Found while investigating #110001.
Also see discussion here.

Describe the problem

When TestNetworkConnectivity in server/storage_api is modified to point to a secondary tenant (e.g. via #110008), the test starts failing when accessing the /_status/connectivity endpoint:

    network_test.go:42: condition failed to evaluate within 45s: from http.go:190: status: 501 Not Implemented, content-type: application/json, body: {
          "error": "method NetworkConnectivity not implemented",
          "code": 12,
          "message": "method NetworkConnectivity not implemented",
          "details": [
          ]
        }, error: <nil>

How to reproduce

from #110008 reset DefaultTestTenant in TestNetworkConnectivity, run the test with a test tenant and observe the failure.

Expected behavior

See discussion here.

• for the KV/storage-level connectivity, once the tenant is granted a suitable capability, the endpoint should work properly and retrieve the storage-level connectivity information using the KV tenant connector over the network.
• we need a design discussion for the connectivity stats between SQL pods.

Epic CRDB-38968

Jira issue: CRDB-31229