Support Producer field defaults

[JacobHayes]

We don't currently allow Producers to set default parameters (1, 2). This was originally to prevent a default value from "subverting" the compute_input_fingerprint calculation, but I think we may be able to relax the checks and still including during fingerprinting.

We should be able to set field defaults safely to Artifact instances, as these will be discoverable by the .inputs property (which is what feeds into dependency tracking), however build/map parameter defaults alone won't be visible so should still be blocked.

One caveat: the @producer setup generates the fields from the function signature - in this case, perhaps when adding fields to __annotations__, we can convert the default value (which would be a python value) to an Artifact instance (using Artifact.cast) and then clear the defaults (build.__defaults__ = None; build.__kwdefaults__ = None).