KIM should validate relevant parts of the shoot spec when Runtime CR is updated

[Disper]

Description
Some fields in Shoot-Spec could be updated independent to the RuntimeCR (e.g. update of Kubernetes images etc.).

We have to agree and implement a strategy how we deal with such cases, also considering if a change to an immutable field (of Shoot-Spec) would be required.

AC:

• Validation. Agree on a strategy how we deal with changes applied on the Shoot-Spec (e.g. Deny modifications to particular fields in Shoot-Spec CR if some values were modified by Gardener, Sync back Shoot-Spec changes to Runtime CR etc. etc.) (e.g. using webhook)
  • Verify the available approaches with it's Pros / Cons and prepare a recommendation for the strategy
  • Share and agree the strategy within the team
  • Should not allow improper modification

[Disper]

Connected to #198

[Disper]

Extracted the actual upgrade (without validation) to a separate issue #270

[tobiscr]

Duplicate of #232