Paired-key Crypto Scheme (#1705)

BEEFY needs two cryptographic keys at the same time. Validators should
sign BEEFY payload using both ECDSA and BLS key. The network will gossip
a payload which contains a valid ECDSA key. The prover nodes aggregate
the BLS keys if aggregation fails to verifies the validator which
provided a valid ECDSA signature but an invalid BLS signature is subject
to slashing.

As such BEEFY session should be initiated with both key. Currently there
is no straight forward way of doing so, beside having a session with
RuntimeApp corresponding to a crypto scheme contains both keys.

This pull request implement a generic paired_crypto scheme as well as
implementing it for (ECDSA, BLS) pair.

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Co-authored-by: Davide Galassi <[email protected]>
Co-authored-by: Robert Hambrock <[email protected]>