Make BEEFY client keystore generic over BEEFY `AuthorityId` type (#2258)

This is the significant step to make BEEFY client able to handle both
ECDSA and (ECDSA, BLS) type signature. The idea is having BEEFY Client
generic on crypto types makes migration to new types smoother.

This makes the BEEFY Keystore generic over AuthorityId and extends its
tests to cover the case when the AuthorityId is of type (ECDSA,
BLS12-377)

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Co-authored-by: Davide Galassi <davxy@datawok.net>
Co-authored-by: Robert Hambrock <roberthambrock@gmail.com>