add openai init go plugin from yukinagae, add vision pro model for ve…

[alonsopf]

• Integrate https://github.com/yukinagae/genkit-go-plugins plugin
• add vertexai pro vision model to be consistent with JS

Checklist (if applicable):

• [ ] Tested (manually, unit tested, etc.)
• [ ] Changelog updated
• [ ] Docs updated

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[apascal07]

Thank you for the contribution but genkit init is going away in 0.9+.