All packages pre-release version 2.0.0-beta.2

Release of all CloudNative.CloudEvents packages version 2.0.0-beta.2

This is the first release since the significant overhaul from 1.x.

See https://github.com/cloudevents/sdk-csharp/blob/master/docs/changes-since-1x.md for more details.

Signed-off-by: Jon Skeet <jonskeet@google.com>