CloudEvents format for messages

[oncicaradupopovici]

Adopt the CNCF Cloud events standard for messaging

https://cloudevents.io/
https://github.com/cloudevents/sdk-csharp

Proposed solution

FeatureFlag: CloudEvents compatible
Keep MessagingEnvelope; Add ToCloudEvent FromCloudEvent functions
Modify MessagingTransport to receive/return MessagingEnvelope instead of ReceiveContext/ SendContext
Add custom ProtocolBinding and Formatter for legacy message format or just use exiting code.
Use existing or develop Formatter/ ProtocolBinding for Stan.
For Rusi Transport, just add enough information related to Cloud Events and let Rusi do the envelope.