Add interfaces for `Client` and `Models`

[reecerussell]

I'm currently working on integrating Gemini/Vertex into my code, but I've had to create a wrapper for the current version of the SDK, to allow me to mock out the client interface for unit testing. To avoid wrapper boilerplate, it would be really helpful to add interfaces for both the Client and Models types, which would enable the use of Moq.

This addition could be easily made without any breaking changes. I'd also happily open a pull request to tackle this, but the contributing guide advised against.

Thanks team!

[shivvaam0001]

Hi @reecerussell
Thank you for your feedback. We appreciate you taking the time to share your thoughts with us, and we'll be filing a feature request.
To help us prioritize this request effectively, any additional details you can provide regarding the impact this feature would have would be very helpful.