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/lib.rs

@@ -27,7 +27,7 @@ pub const SLASHING_PROTECTION_FILENAME: &str = "slashing_protection.sqlite";
 /// The attestation or block is not safe to sign.
 ///
 /// This could be because it's slashable, or because an error occurred.
-#[derive(PartialEq, Debug)]
+#[derive(PartialEq, Debug, Clone)]
 pub enum NotSafe {
     UnregisteredValidator(PublicKeyBytes),
     DisabledValidator(PublicKeyBytes),