[BEEFY] Return valid signatures when verifying commitment (#4259)

Trying to split parts of the
https://github.com/paritytech/polkadot-sdk/pull/1903 into smaller PRs

For https://github.com/paritytech/polkadot-sdk/pull/1903 it would help
if `verify_with_validator_set()` returned the list of valid
authority-signatures pairs, since after the verification we need to send
them in the equivocation proof.