implement data_node._get_user_properties(*key_to_add) method API

[jrobinAV]

For DataNodeviewer core visual element (specifications), a new API should be opened.

It consists in building dynamically and returning the property list containing only the custom properties created by the user. The Taipy internal properties must be filtered out.

Eventually, the caller can provide a list of property keys NOT to filter out.

def (self, *list_of_properties_to_keep): -> Dict
    pass

[trgiangdo]

For now, the implementation of DataNode._get_user_properties() API accepts no parameter, and return only the custom properties of the user.

If it accepts *list_of_properties_to_keep parameter, what will it return?

• The properties in the list + all custom properties.
• Only the properties in the list.

[trgiangdo]

We decided to not add any parameter to the API at the moment. We can do that in the future if necessary