Add restrictions to relation tags in relation extraction

**Is your feature request related to a problem? Please describe.**
Annotating relations and labels in text can become cluttered with lots of incorrect and unnecessary labels.

**Describe the solution you'd like**
One way to cut down on the visual noise is if it was possible to specify that certain relationships can only relate specific types of labels (e.g. in the example [here](https://labelstud.io/templates/relation_extraction) `org:founded_by` should only be allowed to relate `Organisation` -> `Person`. Similarly, other more general restrictions could be useful relating to reflexivity, symmetry etc.