feat: defaults handler for ingress, pods and pvcs

[oliverbaehler]

This Pull Request implements #610

Required some code refactoring to keep code as clean as possible.

Here's some notes for the review:

• The assumption is always, that if a value for Default is defined, it's automatically allowed within a tenant. The default value does not have to be explicit allowed (Better User-Experience).
• Patches are only mutated for Ingress. PVCs and Pods can not be patched on the relevant fields, which would cause a default mutation.
• IngressClass only supported on API networking/v1. Feature supported for Ingress on networking v1 and v1beta1
• StorageClass only supported on API storage/v1
• PriorityClass only supported on API schedule/v1

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[prometherion]

• PVCs and Pods can not be patched on the relevant fields, which would cause a default mutation.

I'm not getting the point here: if we define a mutating webhook for these fields, we should be able to respond with a mutating response that would be executed by the API Server, as we're doing with Namespaces.

Am I missing something else?