Implement Beefy

[tomaka]

paritytech/smoldot#747

[tomaka]

Status update: BEEFY is still a prototype and might still change at any point. Also, all the documentation is old and outdated.

[tomaka]

Latest documentation: https://github.com/paritytech/substrate/blob/master/client/consensus/beefy/README.md

BEEFY votes for the same blocks as GRANDPA, just with a delay, so from a light client perspective we don't actually care in any way about what BEEFY does.

The main change is that the warp sync would now be a "beefy warp sync" instead of a "grandpa warp sync".
The validators would use an ECDSA key for signing the blocks, so we will need to track both the ed25519 and ECDSA key of each validator in order to be able to store these keys in the database.

[tomaka]

The main change is that the warp sync would now be a "beefy warp sync" instead of a "grandpa warp sync".

The initial deployment of BEEFY won't permit that.

[tomaka]

cc paritytech/polkadot-sdk#1118