Add support for new model: Google Gemini

[jravenel]

Currently, OntoChimp only supports the ChatGPT API. We want to add an option to use Google Gemini (Free API: https://ai.google.dev/gemini-api/docs/pricing) as an alternative model. This should include:

• Adding configuration options to switch between ChatGPT and Gemini.
• Refactoring code to allow model selection.
• Implementing Gemini API integration.
• Updating documentation and usage examples.

[jravenel]

@jesudas and @jravenel can explore how to do this, and request @smithGit help, it will help us see if we can understand how to change the code.