Improved Authentication & Configuration

We would like to make the VS Coded extension more robust. We would want to take the OpenAI and Fauxpilot options out and have gitlab and automatically sync with just once click on enable AI Assist

1. The Gitlab › Aiassist: Engine should be removed defaulting to Engine.

2. The Gitlab › Aiassist: Model field seems not to be required and not used. We probably can remove this field as we're going to keep the GitLab engine only.

3. Remove Gitlab › Aiassist: Api Key

4. Remove Gitlab › Aiassist: Server if we plan to stay with the GitLab engine only.

5. Rename Ai Assist to AI Assisted Code Suggestions

6. Readme doc changes

7. Allow to work on non-Gitlab projects?