Add partner model support to vertexai provider

[major]

🚀 Describe the new functionality needed

The recent overhaul of the remote::vertexai provider opens the door for consuming partner models, such as those from Anthropic and Mistral, via the Vertex AI APIs. Most of the existing code can be re-used, but there is some additional conversion necessary since we can't use the regular generate calls that we can with Gemini models.

💡 Why is this needed? What if we don't build it?

This allows users to consume all of the built-in models in Vertex AI as well as the Anthropic/Mistral models. If we don't add this, then users can't use those partner models.

Other thoughts

I'm willing to do the work to build/test this.