Add repository-scoped mutators to transactional RPCs

This MR does some preparatory work to prepare for the world where all or at least most of our repository-scoped mutators will make use of transactions. It does so by introducing a new conditional type -- while currently not really used yet, I'll at a later point add conditionals which depend on feature flags, where each RPC will have its own feature flag.

As the list of RPCs may change at any given time during development, I've added safety checks to verify that the map is in fact up-to-date.