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_metrics/metrics.rs

@@ -80,6 +80,19 @@ lazy_static::lazy_static! {
         "Number of attesters on this host",
         &["task"]
     );
+    /*
+     * Endpoint metrics
+     */
+    pub static ref ENDPOINT_ERRORS: Result<IntCounterVec> = try_create_int_counter_vec(
+        "bn_endpoint_errors",
+        "The number of beacon node request errors for each endpoint",
+        &["endpoint"]
+    );
+    pub static ref ENDPOINT_REQUESTS: Result<IntCounterVec> = try_create_int_counter_vec(
+        "bn_endpoint_requests",
+        "The number of beacon node requests for each endpoint",
+        &["endpoint"]
+    );
 }
 
 pub fn gather_prometheus_metrics<T: EthSpec>(