[builder API] Verify signatures on bids

[realbigsean]

Description

We receive execution payload headers wrapped in BuilderBid objects. These objects include signatures made by the builder. We should verify these signatures are correct so we can retroactively identify bad acting by builders.

We don't yet have this functionality because a local instance of mev-boost will already perform this. But because you can directly connect lighthouse to a relay, we will also want to implement this in lighthouse.

The builder specs describe how these messages are signed.