feat(integration): Add Anthropic Messages.stream support

[tcapelle]

Antrhopic has 2 types of streaming: One by event and another by messages.

We are missing the Messages.stream support, this PR tries to add this. On this mode, the antrhopic library gives you back a ContextManager:

class MessageStreamManager:
    """Wrapper over MessageStream that is returned by `.stream()`.

    ```py
    with client.messages.stream(...) as stream:
        for chunk in stream:
            ...
    ```
    """

    def __init__(
        self,
        api_request: Callable[[], Stream[RawMessageStreamEvent]],
    ) -> None:
        self.__stream: MessageStream | None = None
        self.__api_request = api_request

    def __enter__(self) -> MessageStream:
        raw_stream = self.__api_request()

        self.__stream = MessageStream(
            cast_to=raw_stream._cast_to,
            response=raw_stream.response,
            client=raw_stream._client,
        )

        return self.__stream

    def __exit__(
        self,
        exc_type: type[BaseException] | None,
        exc: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> None:
        if self.__stream is not None:
            self.__stream.close()

I am curious if patching the _IteratorWrapper with this hack:

    def __enter__(self) -> "_IteratorWrapper":
        if hasattr(self._iterator, "__enter__"):
            return self._iterator.__enter__()
        return self

it is a good idea.

[tcapelle]

cc @tssweeney

[circle-job-mirror[bot]]

Preview this PR with FeatureBee: https://beta.wandb.ai/?betaVersion=518d65fd364e6ac4a93a86cb0585e0882faf9f41

[tcapelle]

Ok, now it is working but...

• Had to change some things in the accumulator.py like being able to subclass the _IteratorWrapper
• This is kind of brittle, but I suppose all the integrations are brittle.
• Couldn't find a way to deal with all the possible streaming scenarios that this lib gives

[tcapelle]

I think this is ready for review @tssweeney