Add Payload interface and serialize it

Review Request #10018 — Created June 12, 2018 and submitted — Latest diff uploaded

Information

rb-gateway
master
4dc9b23...

Reviewers

The Payload interface is a generic type that represents the payload of
an event. Since events will have different payloads, we allow them to
define the majority of what they contain and we serialize them
intelligently.

Ran go fmt ./....
Ran go test ./....