Support auction result fetching for namespace filtering

[shenkeyao]

Fetch the auction result from the solver.

The fallback builder should listen for auction results and only build for namespaces that have no auction winner and no reserve sequencer set, regardless of whether the namespaces are registered or not.

This won't be needed if the HotShot requests the builder for specific bundles based on the auction results, which isn't the case for now.

(This is a stretch goal for the first version.)

(Relevant discussion: https://espresso.zulipchat.com/#narrow/stream/440749-Marketplace/topic/Fallback.20Builder.20Building.20for.20Correct.20Namespaces/near/464386365.)