Add OpenAI Gemini Example for VertexAI Notebook

[luxzoli]

Why are these changes needed?

• Add example of how to use Gemini with Autogen using the OpenAI client
• Correct authentication commands with gcloud for environments outside of Google Cloud Shell

Related issue number

Checks

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[olaoluwasalami]

@luxzoli The docs/check(pull_request) is failing, Please can you check this?

[olaoluwasalami]

@luxzoli for the Service Account, instead of the basic role(editor role), can we look at changing it to predefined roles? We can work on this together

[luxzoli]

@luxzoli for the Service Account, instead of the basic role(editor role), can we look at changing it to predefined roles? We can work on this together

Good point, and thanks a lot for the support! The roles/aiplatform.user should actually have all the rights which we need for VertexAI. Of course, for things like impersonation additional permissons are needed, but I guess that is out of scope here

[luxzoli]

@luxzoli The docs/check(pull_request) is failing, Please can you check this?

This should be fixed now :)

[luxzoli]

@luxzoli for the Service Account, instead of the basic role(editor role), can we look at changing it to predefined roles? We can work on this together

Now the Vertex AI User role is used instead