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/beacon_chain/src/lib.rs

@@ -3,7 +3,7 @@ pub mod attestation_verification;
 mod attester_cache;
 mod beacon_chain;
 mod beacon_fork_choice_store;
-mod beacon_proposer_cache;
+pub mod beacon_proposer_cache;
 mod beacon_snapshot;
 pub mod block_reward;
 mod block_times_cache;
@@ -28,6 +28,7 @@ pub mod observed_operations;
 mod persisted_beacon_chain;
 mod persisted_fork_choice;
 mod pre_finalization_cache;
+pub mod proposer_prep_service;
 pub mod schema_change;
 mod shuffling_cache;
 mod snapshot_cache;