Custom RPC for Merkle Mountain Range pallet (#8137)

* Add MMR custom RPC.

* Change RuntimeApi to avoid hardcoding leaf type.

* Properly implement the new RuntimeAPI and wire up RPC.

* Extract Offchain DB as separate execution extension.

* Enable offchain DB access for offchain calls.

* Fix offchain_election tests.

* Skip block initialisation for proof generation.

* Fix integration test setup.

* Fix offchain tests. Not sure how I missed them earlier 🤷.

* Fix long line.

* One more test missing.

* Update mock for multi-phase.

* Address review grumbbles.

* Address review grumbles.

* Fix line width of a comment