🌱 add validation for default, enum and example definitions in variables

[killianmuldoon]

This PR adds schema validation for values defined for default, enum and example in a ClusterClass variable.

e.g.

If a user defines a schema with an integer and minimum value 1 a default, example or enum member of -100 will be found to be invalid and the ClusterClass will be rejected on creation or update.

It also removes the Kubernetes CRD default validation that was part of the code previously.

Fixes #5831

[sbueringer]

I think we should restructure. The current recursion is not ideal (before this PR)

~ something like:

• top-level validateRootSchema func:

  • convertToAPIExtensionsJSONSchemaProps
    • move the basic JSON validation into this func
  • do all the stuff with the Kubernetes schema validators including ValidateDefaulting
  • call validateSchema on the current top-level apiExtensionsSchema

• validateSchema:
  * validate type
  * build a validator with validation.NewSchemaValidator
  * validate Example & Enums
  * call validateSchema recursively for array/object

Let's pair if possible.

[fabriziopandini]

changes lgtm, I will make another past on tests

[sbueringer]

lgtm pending squash

[fabriziopandini]

/lgtm
/approve

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