Adds RabbitMQ bindings

[antiblue]

PR for #310
Adds AMQP 0.9.1 protocol binding using the RabbitMq.Client.

It follows the "RabbitMQ Protocol Binding for CloudEvents" specification.

[jskeet]

Will try to review this next week.

[clemensv]

There is no spec'd protocol binding for AMQP 0.9.1. AMQP 0.9.1 is an expired draft of the AMQP protocol and never made it to standardization. That excludes AMQP 0.9.1 per https://github.com/cloudevents/spec/blob/main/cloudevents/primer.md#qualifying-protocols-and-encodings

What can be done is to submit a binding spec into the "proprietary specs" section for RabbitMQ analog to RocketMQ.
https://github.com/cloudevents/spec/blob/main/cloudevents/proprietary-specs.md

That can then be composed with, but arguably not embedded into the official SDK.

Edit: I only just see that the RMQ binding exists but isn't registered in our proprietary spec list. That should be added. Still skeptical whether a proprietary client should be in the offical SDK.

[antiblue]

Still skeptical whether a proprietary client should be in the offical SDK.

That is a valid point.