Automatic merge from submit-queue (batch tested with PRs 41349, 41532, 41256, 41587, 41657)

client-go: don't import client auth provider packages

Both of these auth providers are useful for kubectl but not so much for everyone importing client-go. Let users optionally import them (example [0]) and reduce the overall number of imports that client-go requires.

Quick grep seems to imply it wont import it after.

```
$ grep -r 'client-go/plugin/pkg/client/auth' staging/
staging/src/k8s.io/client-go/plugin/pkg/client/auth/plugins.go:	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp"
staging/src/k8s.io/client-go/plugin/pkg/client/auth/plugins.go:	_ "k8s.io/client-go/plugin/pkg/client/auth/oidc"
staging/src/k8s.io/client-go/examples/third-party-resources/main.go:	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp"
staging/src/k8s.io/kube-aggregator/pkg/client/clientset_generated/clientset/clientset.go:	_ "k8s.io/client-go/plugin/pkg/client/auth"
staging/src/k8s.io/kube-aggregator/pkg/client/clientset_generated/internalclientset/clientset.go:	_ "k8s.io/client-go/plugin/pkg/client/auth"
```

closes https://github.com/kubernetes/client-go/issues/49
updates https://github.com/kubernetes/client-go/issues/79 (removes cloud.google.com/go import)

cc @kubernetes/sig-api-machinery-pr-reviews @kubernetes/sig-auth-pr-reviews 

```release-notes
client-go no longer imports GCP OAuth2 and OpenID Connect packages by default.
```

[0] https://github.com/kubernetes/client-go/blob/8b466d64c5da37e0d3985b907d812e1f58cb41cb/examples/third-party-resources/main.go#L34-L35