NewGCPStylePublisher will return a pubsub.Publisher that wraps the payload in a GCP pubsub.Message-like object that will make this publisher emulate Google's PubSub posting messages to a server.

NewPublisher will return a pubsub.Publisher that simply posts the payload to the given URL.

GCPPublisher publishes data in the same format as a GCP push-style payload.

Publisher implements the pubsub.Publisher and MultiPublisher interfaces for use in a plain HTTP environment.