Add more checks and logging before genesis (#4730)

## Proposed Changes

This PR adds more logging prior to genesis, particularly on networks that start with execution enabled.

There are new checks using `eth_getBlockByHash/Number` to verify that the genesis state's `latest_execution_payload_header` matches the execution node's genesis block.

The first commit also runs the merge-readiness/Capella-readiness checks prior to genesis. This has two effects:

- Give more information on the execution node's status and its readiness for genesis.
- Prevent the `el_offline` status from being set on `/eth/v1/node/syncing`, which previously caused the VC to complain loudly.

I would like to include this for the Holesky reboot. It would have caught the misconfig that doomed the first Holesky.

## Additional Info

- Geth doesn't serve payload bodies prior to genesis, which is why we use the legacy methods. I haven't checked with other ELs yet.
- Currently this is logging errors with _Capella_ genesis states generated by `ethereum-genesis-generator` because the `withdrawals_root` is not set correctly (it is 0x0). This is not a blocker for Holesky, as it starts from Bellatrix (Pari is investigating).

beacon_node/execution_layer/src/lib.rs

@@ -520,9 +520,9 @@ impl<T: EthSpec> ExecutionLayer<T> {
     ///
     /// This function is a wrapper over `Self::is_synced` that makes an additional
     /// check for the execution layer sync status. Checks if the latest block has
-    /// a `block_number != 0`.
+    /// a `block_number != 0` *if* the `current_slot` is also `> 0`.
     /// Returns the `Self::is_synced` response if unable to get latest block.
-    pub async fn is_synced_for_notifier(&self) -> bool {
+    pub async fn is_synced_for_notifier(&self, current_slot: Slot) -> bool {
         let synced = self.is_synced().await;
         if synced {
             if let Ok(Some(block)) = self
@@ -531,7 +531,7 @@ impl<T: EthSpec> ExecutionLayer<T> {
                 .get_block_by_number(BlockByNumberQuery::Tag(LATEST_TAG))
                 .await
             {
-                if block.block_number == 0 {
+                if block.block_number == 0 && current_slot > 0 {
                     return false;
                 }
             }
@@ -1605,6 +1605,17 @@ impl<T: EthSpec> ExecutionLayer<T> {
         }
     }
 
+    pub async fn get_block_by_number(
+        &self,
+        query: BlockByNumberQuery<'_>,
+    ) -> Result<Option<ExecutionBlock>, Error> {
+        self.engine()
+            .request(|engine| async move { engine.api.get_block_by_number(query).await })
+            .await
+            .map_err(Box::new)
+            .map_err(Error::EngineError)
+    }
+
     pub async fn get_payload_by_hash_legacy(
         &self,
         hash: ExecutionBlockHash,