Deduplicate `VdafConfig` and the `Prio3::new_histogram()` into a builder

[jcjones]

This duplication between the VdafConfig and the Prio3::new_histogram() arguments is a little unfortunate. After we get things working, maybe we should come back here and add convenience constructors/builder methods for well-known VDAF types?

Originally posted by @divergentdave in #4698 (comment)