Propose adding receiver transformer for message queue protocols

[yanmxa]

Some of the event metadata might differ between sender and receiver. Like the Kafka protocol, we expect to add the topic/partition/offset information after receiving the event.

• Sender

Context Attributes,
  specversion: 1.0
  ...
  datacontenttype: application/json
Data,
  {
    "id": 0,
    "message": "Hello, World!"
  }

• Reciever

Context Attributes,
  specversion: 1.0
  ...
  datacontenttype: application/json
Extensions,
  kafkaoffset: 543
  kafkapartition: 0
  kafkatopic: test-confluent-topic
Data,
  {
    "id": 0,
    "message": "Hello, World!"
  }

It works fine when transferring the event with binary mode. But the event metadata become the same between sender and receiver in the structured mode. So I propose adding receiver transformers to modify the event metadata for the receiver.

However, I am concerned about whether this will undermine the original design intent of the structured mode. Therefore, I opened the issue here for discussing. At the same time, to express my ideas more clearly, I also drafted a PR to demonstrate possible code changes: https://github.com/cloudevents/sdk-go/pull/1053