BN Fallback v2 (#2080)

## Issue Addressed

- Resolves #1883

## Proposed Changes

This follows on from @blacktemplar's work in #2018.

- Allows the VC to connect to multiple BN for redundancy.
  - Update the simulator so some nodes always need to rely on their fallback.
- Adds some extra deprecation warnings for `--eth1-endpoint`
- Pass `SignatureBytes` as a reference instead of by value.

## Additional Info

NA

Co-authored-by: blacktemplar <blacktemplar@a1.net>

validator_client/src/http_api/tests.rs

@@ -73,7 +73,9 @@ impl ApiTester {
         config.validator_dir = validator_dir.path().into();
         config.secrets_dir = secrets_dir.path().into();
 
-        let fork_service = ForkServiceBuilder::testing_only(log.clone())
+        let spec = E::default_spec();
+
+        let fork_service = ForkServiceBuilder::testing_only(spec.clone(), log.clone())
             .build()
             .unwrap();
 
@@ -84,7 +86,7 @@ impl ApiTester {
             initialized_validators,
             slashing_protection,
             Hash256::repeat_byte(42),
-            E::default_spec(),
+            spec,
             fork_service.clone(),
             log.clone(),
         );