[FEATURE] Support custom properties for DGML serialization

[chrisc-onaorg]

Is your feature request related to a problem? Please describe. I need to be able to attach property values for nodes and links when serializing to DGML, but right now there's no way to do this, only the ability to provide a collection of property definitions.

Describe the solution you'd like A method of attaching values to nodes and links tied to property objects.

Describe alternatives you've considered The only alternative I can come up with would be to post-process the XML output by the DGML serializer, but with the amount of additional context that would be required I might as well be serializing by hand.

Additional context None.