Add mmr_root() to pallet-mmr API to expose root from state (companion for substrate#11183) (#5276)

* Add mmr_root() to pallet-mmr API to expose root from state

* use the right MmrApi primitives

* bridges: use correct mmr primitives

* rococo: beefy-mmr deposit mmr root digest

* fix lockfile

* update lockfile for {"substrate"}

Co-authored-by: parity-processbot <>