Builder and beacon API updates for Sidecar inclusion proofs

[realbigsean]

• Biggest thing is we're able to completely get rid of blinded vs unblinded blob sidecars, which is a massive simplification.
• The beacon api updates mean can also get rid of the concept of BlindedBlockContents
• This leads to some asymmetry in how we handle blinded and unblinded flows, but it led me to get rid of a bunch of generics in favor of concrete types, and I think it's helping simplify a lot
• Still a WIP, currently compiling but with lots of TODOs and lints to fix