Add support for other AI models

Currently, this extension only supports the gemini-1.5-pro-latest model ([reference](https://github.com/Teygeta/github-pr-extension/blob/4d8ae4db9f53a56252729f93b0da905bb4700bd7/src/content.ts#L134)).

It would be appropriate to find a way to have users choose, in the settings, a model that you want to use and then have the api keys set for that model