All packages version 2.2.0

Release 2.2.0 (with Protobuf 2.0.0-beta.1)

Changes in this release:

- Bug fix: the "source" attribute is now validated to be non-empty
- Bug fix: the JSON event formatters comply with the clarified JSON event format spec
- Dependency: Apache.Avro dependency updated to 1.11.0
- Feature: New package CloudNative.CloudEvents.Protobuf, released as 2.0.0-beta.1

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