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/beacon_node_fallback/Cargo.toml

@@ -10,18 +10,17 @@ path = "src/lib.rs"
 
 [dependencies]
 clap = { workspace = true }
-environment = { workspace = true }
 eth2 = { workspace = true }
 futures = { workspace = true }
 itertools = { workspace = true }
 serde = { workspace = true }
 slot_clock = { workspace = true }
 strum = { workspace = true }
+task_executor = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
 types = { workspace = true }
 validator_metrics = { workspace = true }
 
 [dev-dependencies]
-logging = { workspace = true }
 validator_test_rig = { workspace = true }