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/doppelganger_service/Cargo.toml

@@ -15,6 +15,7 @@ task_executor = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 types = { workspace = true }
+validator_store = { workspace = true }
 
 [dev-dependencies]
 futures = { workspace = true }