Expose MMR root through runtime API - use it in BEEFY client (#11183)

* beefy-gadget: allow custom runtime api provider

* beefy-gadget: use mock runtime api in tests

* pallet-mmr: expose mmr root from state through runtime API

* beefy-gadget: get mmr root from runtime state

* pallet-beefy-mmr: remove MmrRoot from header digests

* frame/mmr: move mmr primitives out of frame

* frame/mmr: completely move primitives out of frame

* address review comments

* beefy-mmr: bring back mmr root from header digest

* clippy fixes for rustc 1.60

* address review comments