Merge pull request #47443 from p0lyn0mial/use_incluster_cfg_when_creating_ext_informers

Automatic merge from submit-queue (batch tested with PRs 48012, 47443, 47702, 47178)

incluster config will be used when creating external shared informers.

**What this PR does / why we need it**:
Previously the loopback configuration was used to talk to the server.
As a consequence a custom API server was unable to talk to the root API server.
This PR changes the above by using incluster configuration to create shared informers.

**Release note**:

```release-note
NONE
```

federation/cmd/federation-apiserver/app/server.go

@@ -102,6 +102,7 @@ func NonBlockingRun(s *options.ServerRunOptions, stopCh <-chan struct{}) error {
 	if err := s.CloudProvider.DefaultExternalHost(s.GenericServerRunOptions); err != nil {
 		return fmt.Errorf("error setting the external host value: %v", err)
 	}
+	s.SecureServing.ForceLoopbackConfigUsage()
 
 	s.Authentication.ApplyAuthorization(s.Authorization)