Spec test sequence trace, a general test vector format for the Ethereum consensus layer

[leolara]

From my experience working on the consensus specs testing. I think it would be a good idea to do here what I describe in the issue below.

In the consensus specs, we need to add a "format" even for each new spec method, and it constrains the type of tests we should implement.

See: ethereum/consensus-specs#4603

I can implement this here.