[Feat]: Getting Resources should return, if the resource has children

**😒 Is your feature request related to a problem? Please describe.**
The frontend renders a TreeView with all resources. If the resource is a folder, it has a chevron-item on the left side, where the user can open the folder to list the notes inside. Currently, this chevron is also shown, if there is no note inside the folder, which could lead to confusion.

**🤔 Describe the solution you'd like**
The resources retrieved should contain a property "hasChildren", which is a boolean indicating if the resource hasChildren.

For notes, this should be always false. For folders, this must be calculated. For the calculation, the column 'parentResourceId' must be searched for the 'resourceId' of the resource, where the property should be attached. The endpoint should only display resources from the logged-in user.