Generic source of options for combo/multi selection field

Currently there is only one combo selection type "repo-or-group". The handling of this is hardcoded in UI so adding a new combo type is impossible without changing the UI = not a pluggable solution.
Solution is to change the combo type to not be related to a specific resource but make type returning the url of a rest resource that can provide possible options values. The rest resource will be expected to return resources of type id/description.