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/config.rs

@@ -10,6 +10,7 @@ use directory::{
 use eth2::types::Graffiti;
 use graffiti_file::GraffitiFile;
 use initialized_validators::Config as InitializedValidatorsConfig;
+use lighthouse_validator_store::Config as ValidatorStoreConfig;
 use sensitive_url::SensitiveUrl;
 use serde::{Deserialize, Serialize};
 use std::fs;
@@ -20,7 +21,6 @@ use tracing::{info, warn};
 use types::GRAFFITI_BYTES_LEN;
 use validator_http_api::{self, PK_FILENAME};
 use validator_http_metrics;
-use validator_store::Config as ValidatorStoreConfig;
 
 pub const DEFAULT_BEACON_NODE: &str = "http://localhost:5052/";