Implement ControlPlane controller for Azure provider

[rfranzke]

Similar to how we have implemented the ControlPlane extension resource controller for the AWS provider let's please now do it for Azure.

Based on the current implementation the ControlPlaneConfig should look like this:

apiVersion: azure.provider.extensions.gardener.cloud/v1alpha1
kind: ControlPlaneConfig
cloudControllerManager:
  featureGates:
    CustomResourceValidation: true

No ControlPlaneStatus needs to be implemented right now (not needed yet).

[rfranzke]

FYI: I'm re-opening this one as it is not yet completely finished (what has been done in gardener-attic/gardener-extensions#94 needs to be implemented for Azure as well). cc @mkoynov

PS: I know that the issue only speaks about the controller and the missing piece is about webhooks, however, I don't want to create to separate ones for tracking the progress with Azure.