Fix slasher conditions

[pawanjay176]

Issue Addressed

Addresses #4900 (comment)

Thanks @jimmygchen

Proposed Changes

Previously, we were adding a BlobSidecar's header to the slasher under the following conditions:

1. Gossip verification fails and the signature is valid
2. Just before importing a fully available block

These conditions do not cover all cases since a valid gossip blob with an equivocating header that gets into the observed_blob_cache will not fail gossip verification and will never be fully available since the proposer need not propagate the full signed equivocating block. Such a BlobSidecar with a slashable block header will just go sit in the DA cache and eventually get pruned.

Hence, this PR adds the signed header before adding the blobs to the DA cache.
I have also changed the BlockQueue data structure to be a HashSet instead of a Vec since we are adding potentially many duplicate headers per slot. Looking at the slasher code, I don't think changing the data structure is an issue, please correct me if that's wrong or unnecessary @michaelsproul

[realbigsean]

good call

[realbigsean]

Nice! I think this just needs a cargo fmt

[michaelsproul]

HashSet is fine for blocks I think, we just process them in a loop here:

lighthouse/slasher/src/slasher.rs

Lines 100 to 106 in 441fc16

	for block in blocks {
	if let ProposerSlashingStatus::DoubleVote(slashing) =
	self.db.check_or_insert_block_proposal(txn, block)?
	{
	slashings.push(*slashing);
	}
	}