See PHI18. Third parties can currently define other types of Drydock operations (like "Merge Check" or "Cherry-Pick") but we won't show them in the UI.

This is a simple change which improves third-party support for now. These kinds of operations generally make sense in the upstream, but the pathways to support are longer.

• Verified that there are no other types of repository operation which we'd want to exclude in the upstream today by reviewing the "Repository Operation" subclasses.
• Will click some buttons in production to make sure this works.