Expose prompt and parser separately

[aaronvg]

Some folks want a more modular approach to using BAML, where they may modify the api request themselves.

This issue tracks exposing the api request baml makes from a function, as well as the parser

[hellovai]

Specifically we could expose this:

# get the raw request BAML would have fired.
prompt = b.prompt.SomeFunction(..)

str = call_llm_yourself(prompt)

# parse into the type using SAP
T = b.parse.SomeFunction(llm_response: str)

# or for streaming
Partial_T = b.stream.parse.SomeFunction(llm_response: str)

This would also enable support for RealTimeAPI and Batch APIs meanwhile while we wait (or any other providers as well).

[PGCodehub]

Yes this would be amazing, make the library more modular