Modularize validator store (#6705)

- Create trait `ValidatorStore` with all functions used by the `validator_services`
- Make `validator_services` generic on `S: ValidatorStore`
- Introduce `LighthouseValidatorStore`, which has identical functionality to the old `ValidatorStore`
- Remove dependencies (especially `environment`) from `validator_services` and `beacon_node_fallback` in order to be able to cleanly use them in Anchor

validator_client/slashing_protection/src/signed_attestation.rs

@@ -10,7 +10,7 @@ pub struct SignedAttestation {
 }
 
 /// Reasons why an attestation may be slashable (or invalid).
-#[derive(PartialEq, Debug)]
+#[derive(PartialEq, Debug, Clone)]
 pub enum InvalidAttestation {
     /// The attestation has the same target epoch as an attestation from the DB (enclosed).
     DoubleVote(SignedAttestation),