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/Cargo.toml

@@ -63,3 +63,4 @@ rand = "0.7.3"
 scrypt = { version = "0.5.0", default-features = false }
 lighthouse_metrics = { path = "../common/lighthouse_metrics" }
 lazy_static = "1.4.0"
+fallback = { path = "../common/fallback" }