Implement updated `update_proposer_boost_root` in fork choice

[jimmygchen]

Description

While updating to consensus-spec-tests v1.7.0-alpha.1 for Gloas (#8688), we encountered failures in fork choice tests on earlier forks:

Failing tests:

• fork_choice_get_head - voting_source_beyond_two_epoch (Altair)
• fork_choice_on_block - similar failures

Root cause:
The spec has been updated here to add a proposer index check before applying proposer boost

• Finishing touches for the gloas fork-choice ethereum/consensus-specs#4807.

The new logic in update_proposer_boost_root (phase0/fork-choice.md) requires:

if is_timely and is_first_block:
  head_state = copy(store.block_states[get_head(store)])
  # ... advance head_state to current slot ...
  if block.proposer_index == get_beacon_proposer_index(head_state):
      store.proposer_boost_root = root

[michaelsproul]

My idea to fix this without requiring access to the head state in fork choice was to use the proposer shuffling root. However, my first attempt at doing this caused more tests to fail, and didn't get the failing tests passing 😅

WIP here: unstable...michaelsproul:lighthouse:proposer-boost-sidechain