Improve onboarding by enabling users to get started via OAuth

Problem to solve

Right now a user has to set his personal access token (PAT) to login to the GitLab instance. So the user needs to go to the instance, go through the settings, grab the token, go back to VSCode and paste the token and hopefully it works. This works but the workflow could be much better here.

Proposal

After trigger the GitLab login command in VSCode

1. the user get asked for the GitLab instance url
2. the browser popup with a tab with the "authorize" application for ...
3. user get some kind of popup that login was successful

This would make it also easier to setup multiple GitLab instances.

Links / references

• Documentation GitLab as an OAuth2 provider
• Discussion: Streamlined GitHub authentication · Issue #93 · microsoft/vscode-pull-request-github

Example of GitHub extension authentication