Add async iteration dunder methods to LiteLLMStreamingAIResponse

[tomusher]

Adds __aiter__ and __anext__ methods to LiteLLMStreamingAIResponse so that the response can properly be iterated over without using the inner stream_wrapper.

Before:

response = await chat_backend.achat(messages=messages, stream=True)

async for chunk in response.stream_wrapper:
    print(chunk)

After:

response = await chat_backend.achat(messages=messages, stream=True)

async for chunk in response:
    print(chunk)