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

[knz]

Found while investigating #110001.

Describe the problem

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

    engine_test.go:51: condition failed to evaluate within 45s: from http.go:190: status: 501 Not Implemented, content-type: application/json, body: {
          "error": "method EngineStats not implemented",
          "code": 12,
          "message": "method EngineStats not implemented",
          "details": [
          ]
        }, error: <nil>

How to reproduce

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

Expected behavior

Once the tenant is granted a suitable capability, the endpoint should work properly. It should retrieve the engine stats over the KV tenant connector over the network.

Epic CRDB-38968

Jira issue: CRDB-31226