Asynchronous text generation streaming

Added a new AsyncTextIteratorStreamer class that allows you to have fully asynchronous text generation streaming in your application. Works identically to TextIteratorStreamer except that you iterate the stream as follows:

generated_text = ""
async for new_text in streamer:
    generated_text += new_text

Requires Python 3.11+ due to use of asyncio.timeout to handle timeout. Falls back to asyncio.wait_for if asyncio.timeout is not available.

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Any hints on how best to implement an async test would be appreciated. :) pytest-asyncio seems to be the way to go...

Who can review?

@gante @ArthurZucker