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/latency.rs

@@ -15,7 +15,7 @@ pub const SLOT_DELAY_DENOMINATOR: u32 = 12;
 pub fn start_latency_service<T: SlotClock + 'static, E: EthSpec>(
     context: RuntimeContext<E>,
     slot_clock: T,
-    beacon_nodes: Arc<BeaconNodeFallback<T, E>>,
+    beacon_nodes: Arc<BeaconNodeFallback<T>>,
 ) {
     let future = async move {
         loop {