Add access to Google Cloud VertexAI SDK for GenAI streaming

[goodrules]

Feature Description

Currently, only the consumer APIs are available via @google/generative-ai-js for Gemini. Would like to include access to the Vertex SDK for the GCP endpoints for Gemini via https://github.com/googleapis/nodejs-vertexai.

Use Case

Developers wanting to leverage their GCP resources to build with Vercel.

Additional context

No response

[williamsmt]

#863 seemed to introduce this but closed?

[lgrammel]

Basic provider (no tool calls / gen ui): https://sdk.vercel.ai/providers/ai-sdk-providers/google-vertex

[lgrammel]

https://github.com/vercel/ai/pull/1775