GitLab support

[almereyda]

Many technical teams work on GitLab, which is another preferred git forge, next to #38 or GitHub.

This issue is left here to track the case of GitLab support in Vrite separately from the other issue.

Steps to pass could be:

• [ ] GitLab OAuth/OIDC login
• [ ] git-sync provider for GitLab in backend/src/lib/git-sync/providers

Looking forward to test this and provide feedback.