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/http_api/src/lib.rs

@@ -4327,7 +4327,8 @@ pub fn serve<T: BeaconChainTypes>(
         .then(
             |task_spawner: TaskSpawner<T::EthSpec>, chain: Arc<BeaconChain<T>>| {
                 task_spawner.spawn_async_with_rejection(Priority::P1, async move {
-                    let merge_readiness = chain.check_merge_readiness().await;
+                    let current_slot = chain.slot_clock.now_or_genesis().unwrap_or(Slot::new(0));
+                    let merge_readiness = chain.check_merge_readiness(current_slot).await;
                     Ok::<_, warp::reject::Rejection>(
                         warp::reply::json(&api_types::GenericResponse::from(merge_readiness))
                             .into_response(),