Keen.IO add-on feature

Hi David Knaack,
I forked your GitHub and add add-on feature into that.
Please review code changes and release it on Nuget, so that developers can easily get add-on feature on SDK.

Add-On documentation: https://keen.io/docs/data-collection/data-enrichment/

For any question or concern, please let me know.

Best Regards,
Rynn Wang