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>

testing/simulator/src/local_network.rs

@@ -9,6 +9,7 @@ use std::sync::Arc;
 use types::{Epoch, EthSpec};
 
 const BOOTNODE_PORT: u16 = 42424;
+pub const INVALID_ADDRESS: &str = "http://127.0.0.1:42423";
 
 /// Helper struct to reduce `Arc` usage.
 pub struct Inner<E: EthSpec> {
@@ -118,6 +119,7 @@ impl<E: EthSpec> LocalNetwork<E> {
         mut validator_config: ValidatorConfig,
         beacon_node: usize,
         validator_files: ValidatorFiles,
+        invalid_first_beacon_node: bool, //to test beacon node fallbacks
     ) -> Result<(), String> {
         let index = self.validator_clients.read().len();
         let context = self.context.service_context(format!("validator_{}", index));
@@ -133,8 +135,12 @@ impl<E: EthSpec> LocalNetwork<E> {
                 .expect("Must have http started")
         };
 
-        validator_config.beacon_node =
-            format!("http://{}:{}", socket_addr.ip(), socket_addr.port());
+        let beacon_node = format!("http://{}:{}", socket_addr.ip(), socket_addr.port());
+        validator_config.beacon_nodes = if invalid_first_beacon_node {
+            vec![INVALID_ADDRESS.to_string(), beacon_node]
+        } else {
+            vec![beacon_node]
+        };
         let validator_client = LocalValidatorClient::production_with_insecure_keypairs(
             context,
             validator_config,