BEEFY + MMR finality relay

[serban300]

Part of #2480

Implement BEEFY + MMR finality relay.

Start from #1372 and make it production-ready.

[serban300]

Opened this issue for better tracking but so far there isn't much progress here.

Things worth mentioning:

• Merged a PR in substrate for generating historical MMR proofs: pallet-mmr: generate historical proofs substrate#12324
• there is an substrate issue in progress for modifying the MMR RPC + Runtime API to use block numbers instead of leaf indices: pallet-mmr: RPC API and Runtime API work with block numbers substrate#12345
• there is another substrate issue for some more polishing for the MMR RPC + Runtime API: pallet-mmr: Consolidate/deduplicate methods in runtime API and RPC API substrate#12391