Companion for: pallet-mmr: move offchain logic to client-side gadget (#6321)

* Spawn MMR gadget when offchain indexing is enabled

* companion PR code review changes: 1st iteration

* Code review changes: 2nd iteration

* update lockfile for {"substrate"}

Co-authored-by: acatangiu <[email protected]>
Co-authored-by: parity-processbot <>