A way to restore the user's session

This commit adds a new public function to the UserService, which allows to set the header and the `CurrentUser` without making any queries.

Useful for when you save the current user before the app suspends and you want to restore the session when the app resumes, without forcing the user to login again.

**TODOs**: 
* No session-expiration is accounted for yet
* It would probably make sense to query for the latest data of the `CurrentUser`