Validation for instance integrations (project_id, group_id, inherit_from_id and instance properties)
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`@bmarjanovic` raised the following idea in this [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179534#note_2320347387:
We should introduce some validation for instance integrations [to validate that `project_id`, `group_id`, and `inherit_from_id` are `nil` and that `instance` is `true`.
issue