Track unrealized justification/finalization in forkchoice

[potuz]

Tracks unrealized justification and finalization in forkchoice. It does not introduce runtime changes. Pure functions with their respective unit tests.

[rauljordan]

After trying to grok some of the unit tests here, I have a concern about the function signature of:

func (f *ForkChoice) InsertOptimisticBlock(
	ctx context.Context,
	slot types.Slot,
	blockRoot, parentRoot, payloadHash [fieldparams.RootLength]byte,
	justifiedEpoch, finalizedEpoch types.Epoch,
) 

It feels very likely someone will mix up the last two function arguments, and even the first three. Highly recommend we switch to a struct of func arguments as an alternative. It will also make unit tests more understandable