[jetbrains] fix git integration

[akosyakov]

Right now a user has to log in with GitHub. It would be nicer if JB backend can fetch GitHub token which is used by a user to authenticate in Gitpod.

Maybe it can be extended to other auth providers like GitLab/BitBucket

It is turned not to be about GitHub integration, but that git does not work. Gitpod integrates on git level via credential helper, but JB backends by default does not rely on it, but require GitHub auth for some reasons. We should reconfigure it in our backend plugin to force JB backend to use git crendetial helper and then hook notifications about missing scopes.