add streaming examples

[pierrevalade]

@guillaq I'm still getting a lot of lint errors on the file streaming_summary.py

I've added documentation on the README.md as well to try to explain more clearly how to stream response, but please double check my work.

[pierrevalade]

the file streaming_summary.py is working 👍

[guillaq]

Fixed the doc

The proper way to get streaming is without the async keyword

@workflowai.agent()
def analyze_call_feedback(input: CallFeedbackInput) -> AsyncIterator[CallFeedbackOutput]:
    ...

Added the following note to the readme:

Note: no need to mark the agent as async here ! It is already asynchronous since it returns an AsyncIterator.
The type checkers some times get confused since they consider that an async function that returns an AsyncIterator is
async twice.
For example, a function with the signature async def foo() -> AsyncIterator[int] may be called
async for c in await foo():... in certain cases...