preprocessors and handlers directly reference preprocessors rather than schemas

[jeffbl]

In the below issue, we note that changing object detection to llm from yolo, via a completely new preprocessor, meant making changes in the later preprocessors and handlers that look specifically for this preprocessory. I think there were some schema issues with referencing the required schema instead, but we should really resolve this, as it breaks the idea that we can swap out object detectors easily.

worst case, we'd likely have to look for multiple preprocessors and accept any from a list. sounds like a mess...

Originally posted by @gvzdv in #1164 (comment)