Migrate gemini and vertex modules to use Google's new genai Python SDK

[willbakst]

Description

Originally mentioned by @Kigstn in #741 (comment)

Looks like Google released a new SDK that provides a unified interface for working with AI Studio and Vertex AI.

We should refactor the gemini module to use this SDK. We can also then essentially completely re-use everything gemini in the vertex module to make the two easier to maintain.

[willbakst]

Looking a little more closely at the SDK, it seems like we could just support Vertex AI through a custom client of the gemini module (thereby making it possible to entirely remove the vertex module, but for backwards compatibility, we should still have the vertex module and just implement it by setting the proper custom client on gemini.call).

[willbakst]

Hmm, upon further thought, this would likely present a breaking change. The short term solution would be to implement a new google module that uses the new types for the new SDK (and leaves the old modules alone so that they are backwards compatible fully).

Then we can mark them as deprecated and update the documentation (and warnings) to point users to the new google module.