Allow adding featuregates

We will need the ability to customize the kubevirt feature gates on the KubeVirt resource.

• https://kubevirt.io/user-guide/operations/activating_feature_gates/

# Example: 
kind: KubeVirt
metadata:
  name: kubevirt
  namespace: kubevirt-system
spec:
  configuration:
    developerConfiguration:
      featureGates:
        - DataVolumes
        - Sidecar
...