Node/Attributes Link Limit

**Is your feature request related to a problem? Please describe.**
Node links can have mutliple outgoing links, despite of the need for it.
Implementationwise the etablishement of a link can be avoided, but this does not stop the user from still draging another one and questioning themself then why the link is not etablished when I still can pull the wire.

**Describe the solution you'd like**
Provide a additional parameter like

```python
dpg.add_node_attribute(
...,
link_limit: int = -1,
...,
)
```

once a number <=1 is supplied, dragging stops when link_limit is reached.

**Describe alternatives you've considered**
In the model layer of the gui, I check the node model, if we have already made a link, and return the etablished link instead of etablish a addtional one.