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

substrate/client/consensus/beefy/Cargo.toml

@@ -39,7 +39,6 @@ sp-consensus-beefy = { path = "../../../primitives/consensus/beefy" }
 sp-core = { path = "../../../primitives/core" }
 sp-crypto-hashing = { path = "../../../primitives/crypto/hashing" }
 sp-keystore = { path = "../../../primitives/keystore" }
-sp-mmr-primitives = { path = "../../../primitives/merkle-mountain-range" }
 sp-runtime = { path = "../../../primitives/runtime" }
 tokio = "1.37"
 
@@ -51,6 +50,7 @@ sc-block-builder = { path = "../../block-builder" }
 sc-network-test = { path = "../../network/test" }
 sp-consensus-grandpa = { path = "../../../primitives/consensus/grandpa" }
 sp-keyring = { path = "../../../primitives/keyring" }
+sp-mmr-primitives = { path = "../../../primitives/merkle-mountain-range" }
 sp-tracing = { path = "../../../primitives/tracing" }
 substrate-test-runtime-client = { path = "../../../test-utils/runtime/client" }