Added support for Gemini 1.5 Flash 8B and updated available versions for Google AI and Vertex AI Gemini models

[chrisraygill]

PR does the following:

• Adds support for Gemini 1.5 Flash 8B to the Google AI plugin
• Adds all available versions to all Gemini models in the Google AI plugin based on https://ai.google.dev/gemini-api/docs/models/gemini
• Adds all available versions to all Gemini models in the Vertex AI plugin based on https://cloud.google.com/vertex-ai/generative-ai/docs/learn/models

Tested Gemini 1.5 Flash 8B manually in the DevUI and it works based on test apps: https://screenshot.googleplex.com/ryTsEwLtx4J2Amc

Checklist (if applicable):

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

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