Refactor formatter interface into stateless function - #708 part 2

[mbleigh]

I needed to refactor this to handle the fact that formatters serve two purposes:

1. They need to be able to configure (overridable) default configuration for the underlying model, things like JSON mode and constrained output.
2. They need to be able to dynamically configure themselves based on a given request for maximum flexibility.

There is now essentially a "static" and "dynamic" aspect to formatters. I'm going with contentType as a model-only signal for the requested return type...by separating it from format we can have formats that build on top of JSON while still relying on it.

I'm trying to chop this up into reasonably reviewable chunks since otherwise this project would turn into a 10K line PR. Apologies if the pacing ends up being a bit strange.

Checklist (if applicable):

• Tested (manually, unit tested, etc.)
• Docs updated