Sync Westend to Millau (#824)

* make finality verifier pallet instantiable

* add second instance of finality verifier pallet to the Millau runtime

* add Westend -> Millau headers relay

* use wss to connect to public westend nodes

* initialize with best_finalized_block

* typo

* Revert "initialize with best_finalized_block"

This reverts commit 954ed2832372d67618abc1a06d47e66faa93f674.

* pass VoterSet by ref

* new bridge initialization code

* loop upper bound

* Polkadot -> Westend

* fixed tests compilation

* default-features

* assert