[Ferature Request] Support Google ModelGarden Models

**Is your feature request related to a problem? Please describe.**
Google offers several 3rd part models through their model garden. Adding support in the genai SDK would be amazing so that users can centralize their usage around a single SDK

**Describe the solution you'd like**
Supporting Google Model Garden models in the GenAI SDK

**Describe alternatives you've considered**
Other approaches are possible, but too custom and brittle to maintain.