Add streaming support to googleai_dart client

[davidmigloz]

Docs: https://ai.google.dev/tutorials/rest_quickstart#stream_generate_content

The schema and main methods from the client are automatically generated from the OpenAPI spec. However, the streaming methods need to be written manually.

To add stream support to generate content we need to:

1. Remove /models/{modelId}:streamGenerateContent from the googleai_openapi_curated.yaml and regenerate the client (as we will add this method manually).
2. Add a streamGenerateContent method in the GoogleAIClient class. You can check OpenAIClient.createCompletionStream as a reference.
3. Write a StreamTransformer that returns valid jsons from the stream of data. You can check _OpenAIStreamTransformer as a reference (but the way the data is sent may be different).

[luisredondo]

I will work on this one! @davidmigloz