Add support for Pub/Sub

[grayside]

Current Behavior

Pub/Sub is suppressed from the code generation process in generate-code.sh because Pub/Sub messages are incompatible with protojson parsing. This is because Pub/Sub messages include duplicate fields message_id/messageId, and publish_time/publishTime, which the protobuf spec indicates is invalid.

Expected Behavior

All Event Sources have type libraries.

Approach

Add a custom function that can be used to pre-process Pub/Sub payloads before unmarshalling.

We already have a function in the repository that can facilitate this that was put in place to enable testing:

google-cloudevents-go/internal/testhelper/prepare.go

Lines 11 to 27 in 322c786

	func PreparePubSubMessagePublishedData(b []byte) ([]byte, error) {
	var j map[string]interface{}
	if err := json.Unmarshal(b, &j); err != nil {
	return nil, fmt.Errorf("json.Unmarshal: %w", err)
	}
	m := j["message"].(map[string]interface{})
	// Remove reserved @type field.
	delete(m, "@type")
	// Remove message_id field name conflict with messageId.
	delete(m, "message_id")
	// Remove publish_time field name conflict with publishTime.
	delete(m, "publish_time")
	j["message"] = m
	return json.Marshal(j)
	}

• Move this function to the public API, preferably into the pubsubdata package. This will require a copying/templating process.
• Unblock Pub/Sub from code generation
• Update the code generator to use this function instead of the internal/testheper package

google-cloudevents-go/generators/protoc-gen-go-googlecetypes/generator.go

Lines 146 to 147 in 322c786

	case params.TypePrefix == "google.events.cloud.pubsub.v1" && dataType == "MessagePublishedData":
	params.PrepareFunction[dataType] = "testhelper.PreparePubSubMessagePublishedData"

[ejcx]

@grayside I noticed this issue open but couldn't figure out how people are meant to parse pub/sub if not with this library? Is there a different library that should be used?