Improve handling of streams using context manager

[jackmpcollins]

OpenAI and Anthropic packages both use context managers to ensure the stream for the streamed response is closed correctly. magentic should do the same!

Add a new Stream (and AsyncStream) class, that is an Iterable with an additional .close() method and ability to be used as a context manager (returning self and automatically calling .close() on exit). This should be usable as a return type on prompt-functions.

StreamedStr and StreamedResponse should also work similarly.

Maybe when the Iterable return type is used, we should return a Stream instance.

Example usage:

from magentic import prompt, StreamedStr


@prompt("Tell me about {country}")
def describe_country(country: str) -> StreamedStr: ...


# Print the chunks while they are being received
with describe_country("Brazil") as stream:
    for chunk in stream:
        print(chunk, end="")
# 'Brazil, officially known as the Federative Republic of Brazil, is ...'