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/mod.rs

@@ -126,6 +126,7 @@ impl<T: EthSpec> MockServer<T> {
             hook: <_>::default(),
             new_payload_statuses: <_>::default(),
             fcu_payload_statuses: <_>::default(),
+            syncing_response: Arc::new(Mutex::new(Ok(false))),
             engine_capabilities: Arc::new(RwLock::new(DEFAULT_ENGINE_CAPABILITIES)),
             _phantom: PhantomData,
         });
@@ -414,14 +415,25 @@ impl<T: EthSpec> MockServer<T> {
         self.ctx
             .new_payload_statuses
             .lock()
-            .insert(block_hash, status);
+            .insert(block_hash, Ok(status));
     }
 
     pub fn set_fcu_payload_status(&self, block_hash: ExecutionBlockHash, status: PayloadStatusV1) {
         self.ctx
             .fcu_payload_statuses
             .lock()
-            .insert(block_hash, status);
+            .insert(block_hash, Ok(status));
+    }
+
+    pub fn set_new_payload_error(&self, block_hash: ExecutionBlockHash, error: String) {
+        self.ctx
+            .new_payload_statuses
+            .lock()
+            .insert(block_hash, Err(error));
+    }
+
+    pub fn set_syncing_response(&self, res: Result<bool, String>) {
+        *self.ctx.syncing_response.lock() = res;
     }
 }
 
@@ -478,8 +490,11 @@ pub struct Context<T: EthSpec> {
     //
     // This is a more flexible and less stateful alternative to `static_new_payload_response`
     // and `preloaded_responses`.
-    pub new_payload_statuses: Arc<Mutex<HashMap<ExecutionBlockHash, PayloadStatusV1>>>,
-    pub fcu_payload_statuses: Arc<Mutex<HashMap<ExecutionBlockHash, PayloadStatusV1>>>,
+    pub new_payload_statuses:
+        Arc<Mutex<HashMap<ExecutionBlockHash, Result<PayloadStatusV1, String>>>>,
+    pub fcu_payload_statuses:
+        Arc<Mutex<HashMap<ExecutionBlockHash, Result<PayloadStatusV1, String>>>>,
+    pub syncing_response: Arc<Mutex<Result<bool, String>>>,
 
     pub engine_capabilities: Arc<RwLock<EngineCapabilities>>,
     pub _phantom: PhantomData<T>,
@@ -489,14 +504,14 @@ impl<T: EthSpec> Context<T> {
     pub fn get_new_payload_status(
         &self,
         block_hash: &ExecutionBlockHash,
-    ) -> Option<PayloadStatusV1> {
+    ) -> Option<Result<PayloadStatusV1, String>> {
         self.new_payload_statuses.lock().get(block_hash).cloned()
     }
 
     pub fn get_fcu_payload_status(
         &self,
         block_hash: &ExecutionBlockHash,
-    ) -> Option<PayloadStatusV1> {
+    ) -> Option<Result<PayloadStatusV1, String>> {
         self.fcu_payload_statuses.lock().get(block_hash).cloned()
     }
 }