Add equivocating blocks to fork choice store

[jtraglia]

Making an issue as a reminder to look into this. @nflaig mentioned it here:

• Do not apply proposer boost to equivocated blocks in Gloas #5535 (comment)

the executable gossip spec could do this now, ie. add equivocating blocks to fork choice store, this is required in several places in fork choice, eg. should_apply_proposer_boost or get_proposer_head

but doing this here seems wrong to me, so the comment says first block with valid signature but at that point, there was no signature check yet

the signature check is done further below on line 651 and also noticed we do the signature check before the proposer check itself, it seems same in lodestar but need to double check why that is

but if we wanna add a equivocating block via on_block it probably has to be done at the end of gossip validation unless we are fine with skipping some gossip checks