Refactor GKE hooks

[moiseenkov]

In order to reduce code duplication and high complexity of the implementation, GKE hooks were refactored in this PR:

1. Existing GKEHook and GKEAsyncHook provide interface to GKE API remain unchanged.
2. The new GKEKubernetesHook(GoogleBaseHook, KubernetesHook) was introduced as a replacement for redundant hooks that used to contain duplicated methods that now are simply inherited from GoogleBaseHook and KubernetesHook:

• GKEDeploymentHook (deprecated)
• GKECustomResourceHook (deprecated)
• GKEPodHook (deprecated)
• GKEJobHook (deprecated)

3. By following the same pattern, the new GKEKubernetesAsyncHook(GoogleBaseAsyncHook, AsyncKubernetesHook) was introduced as a replacement for redundant hooks that used to contain duplicated methods that now are simply inherited from GoogleBaseAsyncHook and AsyncKubernetesHook:

• GKEPodAsyncHook (deprecated)

4. New hooks (GKEKubernetesHook, GKEKubernetesAsyncHook) provide access to the standard Kubernetes API with a GKE authentication.
5. Slightly adjusted system tests (they are all green now).
   
6. Except tiny changes, unit tests remain the same in order to prove stability of the code after refactoring. In the following PR we will disconnect GKE operators from the deprecated hooks, so it will be easy to remove them in the future.

[VladaZakharova]

Hi @potiuk ! Can you please take a look on this PR? This is a blocker for several other PRs for GKE :c

[potiuk]

Sure. Not a complex one.