[WIP] Broadcast Slashing on equivocation

[shyam-patel-kira]

fixes #13088

The PR implements immediate broadcasting of slashing messages when detecting equivocating blocks, helping the network react more quickly to malicious behavior without relying on the full slasher service processing.

[0xalpharush]

this should probably log instead of returning and preventing FCU? could not check for block equivocation seems more accurate

[0xalpharush]

I think this should probably be done in the gossip validation of beacon blocks for two reasons:

1. I may not understand the spec properly but I believe signing equivocating blocks even if they're invalid should bet met with a slashing proposal
2. There is probably no need for the slashing service to be sent a block which was already deemed equivocating and resulted in a slashing proposal being broadcast
   

   prysm/beacon-chain/sync/validate_beacon_blocks.go

   Lines 84 to 95 in 30a136f

   	if features.Get().EnableSlasher {
   	// Feed the block header to slasher if enabled. This action
   	// is done in the background to avoid adding more load to this critical code path.
   	go func() {
   	blockHeader, err := interfaces.SignedBeaconBlockHeaderFromBlockInterface(blk)
   	if err != nil {
   	log.WithError(err).WithField("blockSlot", blk.Block().Slot()).Warn("Could not extract block header")
   	return
   	}
   	s.cfg.slasherBlockHeadersFeed.Send(blockHeader)
   	}()
   	}

Fwiw I am just an interested bystander using this as a learning opportunity. Feel free to ignore or let me know if this is unhelpful