feat: remove the VS Code Auth provider in favor of custom OAuth flow

mentioned in merge request gitlab-org/remote-development/gitlab-workspaces-tools!8 (closed)

mentioned in merge request gitlab-org/workspaces/gitlab-workspaces-tools!3 (merged)

mentioned in merge request gitlab-org/workspaces/gitlab-workspaces-tools!12 (closed)

mentioned in merge request gitlab-org/workspaces/gitlab-workspaces-tools!26 (merged)

mentioned in merge request gitlab-org/workspaces/gitlab-workspaces-tools!28 (closed)