Create better abstraction for LLMRequest converesions

[joshgreaves]

Right now all LLMRequest converters live in request.py (see #59 where it was introduced).

There is probably a better pattern to do something like:

class LLMRequestConverter(Protocol[RequestType]):
  def to_ares_llm_request_format(self, request: RequestType) -> LLMRequest:
    ...

  def from_ares_llm_request_format(self, request: LLMRequest) -> RequestType:
    ...

And implement one for each of chat completions, responses, and messages formats.