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/src/notifier.rs

@@ -1,4 +1,5 @@
 use crate::{DutiesService, ProductionValidatorClient};
+use lighthouse_validator_store::LighthouseValidatorStore;
 use metrics::set_gauge;
 use slot_clock::SlotClock;
 use tokio::time::{sleep, Duration};
@@ -32,7 +33,9 @@ pub fn spawn_notifier<E: EthSpec>(client: &ProductionValidatorClient<E>) -> Resu
 }
 
 /// Performs a single notification routine.
-async fn notify<T: SlotClock + 'static, E: EthSpec>(duties_service: &DutiesService<T, E>) {
+async fn notify<T: SlotClock + 'static, E: EthSpec>(
+    duties_service: &DutiesService<LighthouseValidatorStore<T, E>, T>,
+) {
     let (candidate_info, num_available, num_synced) =
         duties_service.beacon_nodes.get_notifier_info().await;
     let num_total = candidate_info.len();