Fallback building v2

[mempirate]

Context

Our current fallback block building mechanism is not very efficient in that it proposes only the commitments and nothing more or less. It would be good to revisit local building in collaboration with the local execution client and some other mechanisms (maybe and engine API that supports ILs) when this comes out.

If local building becomes sufficiently good, we might even have a block value comparison vs. PBS and have a min-bid option inside of the sidecar.