feat(py): Core VertexAI plugin

[Irillit]

This is a small pull request to correct VertexAI. I aim to provide basic corrections here to add more sophisticated features later.

• I previously created draft and I've used the feedback from there.

Checklist (if applicable):

• Moved the code from init
• Separated Gemini model family, defined versions
• Added retrieval of GCP projectId and location
• Tested manually

[google-cla]

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[Irillit]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

With newer Ubuntu I used wrong email. Now it is fixed.

[kirgrim]

LGTM!