Add selective code owner removal support

Closes #1270 (closed)

In addition to adding the attribute, this MR will also migrate gitlab_project_level_mr_approvals to be a framework resource instead of an SDK resource. That's because the validators from the framework allow you to validate the value against the value of other attributes in the config, whereas the SDK can only validate using the one attribute.

That means implementing this check in the SDK would fail at apply-time, whereas migrating it to a framework allows us to make it fail at plan time properly.