Automatic merge from submit-queue (batch tested with PRs 48224, 45431, 45946, 48775, 49396)

[client-go] Fake Dynamic Client

This commit converts `"k8s.io/client-go/dynamic".Client` to an interface, and implements fake versions of both `ClientPool` and `Client`.  This allows components which make uses of these clients to be tested in the same way that clientset-based components can be tested, using the standard `testing.Fake` machinery.

**Release note**:
```release-note
NONE
```