Add eip_3076 crate (#8206)

#7894


  Moves the `Interchange` format from `slashing_protection` and thus removes the dependency on `slashing_protection` from `eth2` which can now just depend on the slimmer `eip_3076` crate.


Co-Authored-By: Mac L <mjladson@pm.me>

validator_client/slashing_protection/src/interchange_test.rs

@@ -1,8 +1,8 @@
 use crate::{
     SigningRoot, SlashingDatabase,
-    interchange::{Interchange, SignedAttestation, SignedBlock},
     test_utils::{DEFAULT_GENESIS_VALIDATORS_ROOT, pubkey},
 };
+use eip_3076::{Interchange, SignedAttestation, SignedBlock};
 use serde::{Deserialize, Serialize};
 use std::collections::HashSet;
 use tempfile::tempdir;