[Merge] Optimistic Sync: Stage 1 (#2686)

* Add payload verification status to fork choice

* Pass payload verification status to import_block

* Add valid back-propagation

* Add head safety status latch to API

* Remove ExecutionLayerStatus

* Add execution info to client notifier

* Update notifier logs

* Change use of "hash" to refer to beacon block

* Shutdown on invalid finalized block

* Tidy, add comments

* Fix failing FC tests

* Allow blocks with unsafe head

* Fix forkchoiceUpdate call on startup

beacon_node/beacon_chain/src/errors.rs

@@ -136,6 +136,9 @@ pub enum BeaconChainError {
     AltairForkDisabled,
     ExecutionLayerMissing,
     ExecutionForkChoiceUpdateFailed(execution_layer::Error),
+    HeadMissingFromForkChoice(Hash256),
+    FinalizedBlockMissingFromForkChoice(Hash256),
+    InvalidFinalizedPayloadShutdownError(TrySendError<ShutdownReason>),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);