Prepare proposer (#3043)

## Issue Addressed

Resolves #2936

## Proposed Changes

Adds functionality for calling [`validator/prepare_beacon_proposer`](https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Validator/prepareBeaconProposer) in advance.

There is a `BeaconChain::prepare_beacon_proposer` method which, which called, computes the proposer for the next slot. If that proposer has been registered via the `validator/prepare_beacon_proposer` API method, then the `beacon_chain.execution_layer` will be provided the `PayloadAttributes` for us in all future forkchoiceUpdated calls. An artificial forkchoiceUpdated call will be created 4s before each slot, when the head updates and when a validator updates their information.

Additionally, I added strict ordering for calls from the `BeaconChain` to the `ExecutionLayer`. I'm not certain the `ExecutionLayer` will always maintain this ordering, but it's a good start to have consistency from the `BeaconChain`. There are some deadlock opportunities introduced, they are documented in the code.

## Additional Info

- ~~Blocked on #2837~~

Co-authored-by: realbigsean <seananderson33@GMAIL.com>

beacon_node/http_api/src/lib.rs

@@ -2229,6 +2229,13 @@ pub fn serve<T: BeaconChainTypes>(
                             )
                         })?;
 
+                    chain.prepare_beacon_proposer_blocking().map_err(|e| {
+                        warp_utils::reject::custom_bad_request(format!(
+                            "error updating proposer preparations: {:?}",
+                            e
+                        ))
+                    })?;
+
                     Ok(())
                 })
             },