BEEFY: Define basic fisherman (#4328)

Related to https://github.com/paritytech/polkadot-sdk/pull/1903

For #1903 we will need to add a Fisherman struct. This PR:
- defines a basic version of `Fisherman` and moves into it the logic
that we have now for reporting double voting equivocations
- splits the logic for generating the key ownership proofs into a more
generic separate method
- renames `EquivocationProof` to `DoubleVotingProof` since later we will
introduce a new type of equivocation

The PR doesn't contain any functional changes