Implement el_offline and use it in the VC (#4295)

## 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.

beacon_node/execution_layer/src/test_utils/handle_rpc.rs

@@ -30,7 +30,12 @@ pub async fn handle_rpc<T: EthSpec>(
         .map_err(|s| (s, GENERIC_ERROR_CODE))?;
 
     match method {
-        ETH_SYNCING => Ok(JsonValue::Bool(false)),
+        ETH_SYNCING => ctx
+            .syncing_response
+            .lock()
+            .clone()
+            .map(JsonValue::Bool)
+            .map_err(|message| (message, GENERIC_ERROR_CODE)),
         ETH_GET_BLOCK_BY_NUMBER => {
             let tag = params
                 .get(0)
@@ -145,7 +150,9 @@ pub async fn handle_rpc<T: EthSpec>(
 
             // Canned responses set by block hash take priority.
             if let Some(status) = ctx.get_new_payload_status(request.block_hash()) {
-                return Ok(serde_json::to_value(JsonPayloadStatusV1::from(status)).unwrap());
+                return status
+                    .map(|status| serde_json::to_value(JsonPayloadStatusV1::from(status)).unwrap())
+                    .map_err(|message| (message, GENERIC_ERROR_CODE));
             }
 
             let (static_response, should_import) =
@@ -320,11 +327,15 @@ pub async fn handle_rpc<T: EthSpec>(
 
             // Canned responses set by block hash take priority.
             if let Some(status) = ctx.get_fcu_payload_status(&head_block_hash) {
-                let response = JsonForkchoiceUpdatedV1Response {
-                    payload_status: JsonPayloadStatusV1::from(status),
-                    payload_id: None,
-                };
-                return Ok(serde_json::to_value(response).unwrap());
+                return status
+                    .map(|status| {
+                        let response = JsonForkchoiceUpdatedV1Response {
+                            payload_status: JsonPayloadStatusV1::from(status),
+                            payload_id: None,
+                        };
+                        serde_json::to_value(response).unwrap()
+                    })
+                    .map_err(|message| (message, GENERIC_ERROR_CODE));
             }
 
             let mut response = ctx