## Issue Addressed
Closes https://github.com/sigp/lighthouse/issues/4291, part of #3613.
## Proposed Changes
- Implement the `el_offline` field on `/eth/v1/node/syncing`. We set `el_offline=true` if:
- The EL's internal status is `Offline` or `AuthFailed`, _or_
- The most recent call to `newPayload` resulted in an error (more on this in a moment).
- Use the `el_offline` field in the VC to mark nodes with offline ELs as _unsynced_. These nodes will still be used, but only after synced nodes.
- Overhaul the usage of `RequireSynced` so that `::No` is used almost everywhere. The `--allow-unsynced` flag was broken and had the opposite effect to intended, so it has been deprecated.
- Add tests for the EL being offline on the upcheck call, and being offline due to the newPayload check.
## Why track `newPayload` errors?
Tracking the EL's online/offline status is too coarse-grained to be useful in practice, because:
- If the EL is timing out to some calls, it's unlikely to timeout on the `upcheck` call, which is _just_ `eth_syncing`. Every failed call is followed by an upcheck [here](693886b941/beacon_node/execution_layer/src/engines.rs (L372-L380)), which would have the effect of masking the failure and keeping the status _online_.
- The `newPayload` call is the most likely to time out. It's the call in which ELs tend to do most of their work (often 1-2 seconds), with `forkchoiceUpdated` usually returning much faster (<50ms).
- If `newPayload` is failing consistently (e.g. timing out) then this is a good indication that either the node's EL is in trouble, or the network as a whole is. In the first case validator clients _should_ prefer other BNs if they have one available. In the second case, all of their BNs will likely report `el_offline` and they'll just have to proceed with trying to use them.
## Additional Changes
- Add utility method `ForkName::latest` which is quite convenient for test writing, but probably other things too.
- Delete some stale comments from when we used to support multiple execution nodes.
## Issue Addressed
NA
## Proposed Changes
- Removes a duplicated log in the fallback code for the VC.
- Updates the text in the remaining de-duped log.
## Additional Info
Example
```
Dec 23 05:19:54.003 WARN Beacon node is syncing endpoint: http://xxxx:5052/, head_slot: 88224, sync_distance: 161774
Dec 23 05:19:54.003 WARN Beacon node is not synced endpoint: http://xxxxx:5052/
```
## 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>
