Add google-vertex-openapi provider for third-party models (GLM) #10304

Problem

The existing google-vertex provider uses the official SDK and only supports Google's native models (Gemini). Third-party models like GLM-4.7 from ZhipuAI are available on Vertex AI but require access through the OpenAPI-compatible endpoint with different authentication (gcloud ADC or service account instead of the official SDK).

Requested Solution

Add a new google-vertex-openapi provider that:

1. Uses the OpenAPI-compatible endpoint on Vertex AI
2. Supports Application Default Credentials (ADC) via google-auth-library
3. Works with "gcloud auth application-default login" or service account authentication
4. Allows configuring project ID and location via config or environment variables
5. Is configurable for third-party models like GLM-4.7

Use Cases

• Users with GLM-4.7 models deployed on Vertex AI
• Other third-party models available via Vertex Model Garden
• Users who prefer gcloud ADC authentication over service account management

Alternative Solutions Considered

• Use the existing google-vertex provider: Does not work for third-party models
• Add models directly to models.dev API: Requires API access
• Create an auth plugin: Plugins cannot add new providers, only modify existing ones

Additional Context

This follows the existing custom loader pattern in provider.ts.