✨ Make Clusterclass webhook Cluster-aware

[killianmuldoon]

Add cluster aware checks to the clusterClass webhook that:

1. Disallow clusterClass deletion if any clusters using the clusterClass still exist
2. Allow removing a MachineDeploymentClass from a Cluster as long as the MachineDeploymentClass is not in use by an existing MachineDeployment does not exist.

Previously:

• any clusterclass would be deleted even if it was in use.
• MachineDeploymentClasses were not ever allowed to be removed from an existing clusterClass.

Note this is built on top of #5318 and should not be merged until after that PR is merged. Changes introduced in this PR are in the second commit.

Fixes: #5498

[sbueringer]

Please let me know when I should do another review and please resolve the straightforward findings (i.e. the conversations) which have been fixed.

[sbueringer]

In this case we will now have an apierror wrapped into an field error wrapped into an apierror. I'm not sure what the error for users will look like.

My intention was to suggest the following:

• return a regular error in getClustersUsingClusterClass
• ValidateDelete:
  • Take the error and wrap it into an apierror in ValidateDelete
• ValidateCreate/Update
  • Call getClustersUsingClusterClass directly in validate instead of in validateRemovedMachineDeploymentClassesAreNotUsed, if there is an error wrap it into an apierror and return it
    • If I understood it correctly we have to move getClustersUsingClusterClass that level up anyway sooner or later to avoid calling this func multiple times.

WDYT?

[killianmuldoon]

I think we're going to have to wrap the client error into a field.ErrorList at that level anyway in order to retain the errors from earlier in validate. I'll update with what I have, but I don't see any direct and clean way to wrap all of these errors into something that will be perfect in every case.

[sbueringer]

I'm not sure but when getClustersUsingClusterClass is called directly in validate I would have thought that we can just return the error we want, but maybe I'm missing something

[killianmuldoon]

Validate will already have some errors wrapped in field.ErrorList by the time getClustersUsingClusterClass is called so we should wrap that error in the same way.

[sbueringer]

As discussed, we now have one api error which contains a list of field errors and that's fine (as we still want to return the field errors)

[sbueringer]

/retest

[sbueringer]

/lgtm

[fabriziopandini]

/lgtm
/approve

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