Started work to make it possible to pass in per-model reasoning config
Cleaned up how we instantiate RequestParameters (TBD: remove
RequestParameters?)
This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.
