pallet-mmr: move offchain logic to client-side gadget (#12753)

* Move MMR utils methods from pallet to primitives

Signed-off-by: Serban Iorga <serban@parity.io>

* Add method to MmrApi

* Move forks expanding logic from babe to primitives

* Implement MMR gadget

* Remove prunning logic from the MMR pallet

* Code review changes: 1st iteration

* Replace MaybeCanonEngine with CanonEngineBuilder

* fix mmr_leaves_count() for kitchen sink demo

* Update client/merkle-mountain-range/src/canon_engine.rs

Co-authored-by: Adrian Catangiu <adrian@parity.io>

* Code review changes: 2nd iteration

* fix INDEXING_PREFIX

* impl review comments

* add documentation and minor rename

Signed-off-by: Serban Iorga <serban@parity.io>
Co-authored-by: Adrian Catangiu <adrian@parity.io>