Spec v1.7.0 alpha.3 (#8988)

Update spec code for compliance with spec v1.7.0-alpha.3: https://github.com/ethereum/consensus-specs/releases/tag/v1.7.0-alpha.3

The actual consensus changes are minimal. There are few more changes that are only relevant to fork choice or P2P validation that we will pick up in future PRs.

The change "Ignore beacon block if parent payload unknown" is currently covered in a hacky way by `load_parent` and can be improved once we have fork choice.

The change "Add parent_block_root to bid filtering key" is relevant to bid gossip validation, which we don't have at all in unstable yet.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/block_verification.rs

@@ -1940,6 +1940,7 @@ fn load_parent<T: BeaconChainTypes, B: AsBlock<T::EthSpec>>(
             {
                 if block.as_block().is_parent_block_full(parent_bid_block_hash) {
                     // TODO(gloas): loading the envelope here is not very efficient
+                    // TODO(gloas): check parent payload existence prior to this point?
                     let envelope = chain.store.get_payload_envelope(&root)?.ok_or_else(|| {
                         BeaconChainError::DBInconsistent(format!(
                             "Missing envelope for parent block {root:?}",