refactor: fork choice store

[unnawut]

I'm not too confident I get this right at all but creating this issue for tracking & discussion...

Fork choice store is carrying a lot of responsibilities that are beyond fork choice decision and we should move these out of it:

• forkchoice.store._validate_block_signatures() -> should already be validated before entering fork choice
  • Done by block: mv verify_signatures to SignedBlockWithAttestation impl #147
• forkchoice.store._process_block_body_attestations() -> fork choice should only know of attestations through store.on_attestation(), maybe triggered by state.process_attestation()
  • Done by SignedBlockWithAttestation: add process_block_body_attestations #153
• forkchoice.store._process_proposer_attestation() -> fork choice should only know of attestations through store.on_attestation(), maybe triggered by state.process_block()
  • Done by store: rm useless _process_proposer_attestation #155
• produce_block_with_signatures() -> fork choice should only need to provide get_proposal_head()
• produce_attestation() -> fork choice should only need to provide get_attestation_target()
  • Done by containers: mv produce_attestation to Validator #188

Thanks to @jihoonsong for raising this earlier in group chats

[tcoratger]

forkchoice.store._validate_block_signatures() -> should already be validated before entering fork choice

@unnawut This is solved in #147 no?

[tcoratger]

@unnawut @jihoonsong #188 should fix the point about
produce_attestation() -> fork choice should only need to provide get_attestation_target()