Provide a safer interface to setPlanFnOutputs

[ag-eitilt]

Filtering out uninteresting files in the Plan/Job itself rather than only after getting an allOutputs : List Path has some promising implications for hashing optimization, database management, etc., but currently the only hook for that is FnOutputs: List String => List String which is a particularly dangerous function to use outside initially defining a runner. We should provide a OutputsFilter: String => Boolean entry point which makes the interface a bit less of a footgun.

Past discussions have talked about implementing this on top of the FnOutputs field, but if we fully separate it into a first-class member, we'd be able to leave FnOutputs as detecting "all files created by the Job" while OutputsFilter would be "all interesting files created by the Job", and the difference between the two provides a very easy determination of "supplemental files" which would need to be cleaned but not hashed/cached/etc.

(I know this keeps coming up in meetings, but I didn't see any issue tracking it.)