Please add Notifications interface to C# header file

[BOLL7708]

The interface for Notifications is missing in the C# header, I've been using a library where the header file was edited. It would be really nice if this could be added officially so we don't have to edit the header for each release of OpenVR.

This is a gist of what to add to the most recent header file, based on the modifications done in the library mentioned. As it's a generated file there's not really a point to making a pull request.

I've mentioned this as a comment in a different thread quite some time ago, but figured to have something happen I'll post about it specifically. 🤞

[aleiby]

Thanks, I've updated the source so this should show up in the next SDK or the one after that. Also added the missing VRIOBuffer accessor.