Add validator spec

[ralexstokes]

Fixes #13.

[lightclient]

Overall looks really great; thank you @ralexstokes.

[terencechain]

lgtm!

[mkalinin]

What do you think about including fallback to a local build process to the steps outlined above? In this case the place from which the fallback becomes dangerous (due to slashing) could be clearly defined.

[ralexstokes]

I think it would obscure the flow outlined above but I'm open to it if many think it is clearer.

As far as I know most clients have some version of this implemented already and they have two distinct code paths so it maps directly to say "run these two tasks, race them against a deadline"

[mkalinin]

I am fine with current description as long as implementers clearly understand the last moment where they can safely sign off on a payload that is built locally.

[tbenr]

maybe it could be clearer if we refer BUILDER_PROPOSAL_DELAY_TOLERANCE specifically to bid providing.
something like

... will give the builders a duration of BUILDER_PROPOSAL_DELAY_TOLERANCE to provide a SignedBuilderBid before the external builder is considered to have hit the deadline and the external

or also rename it to BUILDER_BID_PROPOSAL_DELAY_TOLERANCE?

this way it is clear the assumption that the fallback is no more an option once validator sign a blind block made with the bid.

[ralexstokes]

ok I have addressed the feedback so far, unless there is more feedback soon I say we go ahead and merge it in

[lightclient]

LGTM