CloudEvent should expose a method to get all of the CE headers

[lance]

See: https://github.com/knative/docs/pull/2446/files#diff-de16006ea59029f2429d3228fed269c6R46-R55

The SDK should provide a simpler way to extract all of the headers from a CE. Check with sdk-go to see if/how this is exposed there. I'm not 100% sure about exposing them on a CloudEvent directly since that binds it to the HTTP transport. But what happens when we implement AVRO for example?

[lance]

In retrospect, it seems that adding this method directly to the CloudEvent class is perhaps not the best approach. That conflates the event itself with the HTTP transport protocol. I would like to keep these two concepts decoupled. I currently have an implementation for this on the HTTPEmitter which is introduced in this PR soon #141. Once that PR lands, I'll submit this change for consideration.