VertexAI Streaming support

[ADR-007]

Hello,

I see that streaming is not supported by instructor for VertexAI, while it is very simple to fix:

instructor/dsl/partial.py:

class PartialBase(Generic[T_Model]):
    # ...
    @staticmethod
    async def extract_json_async(
        completion: AsyncGenerator[Any, None], mode: Mode
    ) -> AsyncGenerator[str, None]:
        async for chunk in completion:
            try:
                # ...
                elif mode == Mode.VERTEXAI_JSON:
                    yield chunk.candidates[0].content.parts[0]
                # ...

Currently, I'm using this mixin with my models as a workaround:

class VertexAiStreamingModelMixin:
    @staticmethod
    async def extract_json_async(
        completion: AsyncGenerator[Any, None], mode: Mode
    ) -> AsyncGenerator[str, None]:
        # Gemini Streaming is not implemented on the instructor library yet
        # So, we need to override PartialBase.extract_json_async in
        # .venv/lib/python3.12/site-packages/instructor/dsl/partial.py
        if mode == Mode.VERTEXAI_JSON:
            async for chunk in completion:
                yield chunk.candidates[0].content.parts[0].text
        else:
            async for chunk in super().extract_json_async(completion, mode):
                yield chunk

I'm using VertexAI with Gemini Pro 1.5

It would be awesome to have it in the library!
Thanks!