GitHub OAuth App

This extension has a necessary login, implemented with a GitHub OAuth App.
The creation and use of this app is defined in [GitHub's documentation](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/creating-an-oauth-app).
The app is currently registered under my account, which can be either transferred (feel free to ask me) or simply recreated under for example Theia GitHub enterprise.

1. When creating the app, we provide a name and a redirect Uri. Then, we receive a client id and a client secret.
_This might be a problem when issue #55 is fixed, can we pass a list of redirect Uris?_

2. In our code frontend, we define in the file `extensions/ai-assistant-vsc/src/ressource/loginScript.js` the variables `scope`, `redirectUri` and `clientId`. _The scope defines what data we will access from GitHub token, see [doc](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/scopes-for-oauth-apps)._

3. In the backend, in the file `backend/secrets.env` (which is not pushed to git) we store the client secret as `CLIENT_SECRET=[...]`.