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/Cargo.toml

@@ -22,6 +22,7 @@ fdlimit = "0.3.0"
 graffiti_file = { workspace = true }
 hyper = { workspace = true }
 initialized_validators = { workspace = true }
+lighthouse_validator_store = { workspace = true }
 metrics = { workspace = true }
 monitoring_api = { workspace = true }
 parking_lot = { workspace = true }