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/http_metrics/Cargo.toml

@@ -6,6 +6,7 @@ authors = ["Sigma Prime <contact@sigmaprime.io>"]
 
 [dependencies]
 health_metrics = { workspace = true }
+lighthouse_validator_store = { workspace = true }
 lighthouse_version = { workspace = true }
 logging = { workspace = true }
 malloc_utils = { workspace = true }
@@ -17,6 +18,5 @@ tracing = { workspace = true }
 types = { workspace = true }
 validator_metrics = { workspace = true }
 validator_services = { workspace = true }
-validator_store = { workspace = true }
 warp = { workspace = true }
 warp_utils = { workspace = true }