Beefy client generic on aduthority Id (#1816)

Revived version of https://github.com/paritytech/substrate/pull/13311 .
Except Signature is not generic and is dictated by AuthorityId.

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Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Robert Hambrock <roberthambrock@gmail.com>
Co-authored-by: Adrian Catangiu <adrian@parity.io>