Support for Feedback Events

[jamescrosswell]

Originally posted by @jamescrosswell in #3604

The .NET SDK is still implementing User Feedback using a USER_REPORT_ENVELOPE which means feedback has to be associated with exception events (an eventid is required in the envelope).

Since then a new Feedback Events format has been introduced that allows User Feedback to be sent independently of exceptions. The .NET SDK. should also support Feedback Events.