Refactor policy validation on create and update, including tests

[viccuad]

We need to refactor and cleanup webhooks validation, as we realized that we could increase readability and test coverage

Acceptance criteria

• The file api/policies/v1/policy_validation_matchconditions.go was copy-pasted from k8s.io/kubernetes (see license, copyright, comment at its beginning). We tried to import as-is, but it makes more sense to own it instead. For that:
  • Enable linter in the file (check golangci-lint.yml).
  • Consider moving its functions into api/policies/v1/policy_validation.go and perform any cleanup necessary.
• Factories should be refactored with the builder pattern.
• Add integration tests for the webhooks methods: ValidateCreate, ValidateDelete, and ValidateUpdate.
  ATM We test each validator in isolation (see policy_validation_test.go), however, we do not test that the validators are correctly integrated into the webhooks. We could for instance add just a couple of test cases: one with a valid resource and one with a resource that all fields in an invalid state, so that we are sure we are passing through the right validators.