organize essential APIs into platform, workflow and pkg folders (#337)

Add more logic to the way code is organized.

	/pkg -- library code not directly connected to micromdm
	/mdm -- packages meant for the services devices interract with. The MDM protocol.
	/dep -- DEP API and related packages.
	/platform -- Core APIs the server provides. Commands API, Devices API, queue, pubsub etc.
	/workflow -- Packages/API that build on top of platform. Today that's the webhook package.
		     Depending on what ends up here, the workflow folder might become its own repository.

workflow/webhook/command.go

@@ -0,0 +1,60 @@
+package webhook
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/micromdm/micromdm/mdm/connect"
+	"github.com/micromdm/micromdm/platform/pubsub"
+	"github.com/pkg/errors"
+)
+
+const contentType = "application/x-apple-aspen-mdm"
+
+type CommandWebhook struct {
+	Topic       string
+	CallbackURL string
+	HTTPClient  *http.Client
+}
+
+func NewCommandWebhook(httpClient *http.Client, topic, callbackURL string) (*CommandWebhook, error) {
+	if topic == "" {
+		return nil, errors.New("webhook: topic should not be empty")
+	}
+
+	if callbackURL == "" {
+		return nil, errors.New("webhook: callbackURL should not be empty")
+	}
+
+	return &CommandWebhook{HTTPClient: httpClient, Topic: topic, CallbackURL: callbackURL}, nil
+}
+
+func (cw CommandWebhook) StartListener(sub pubsub.Subscriber) error {
+	connectEvents, err := sub.Subscribe(context.TODO(), "commandWebhook", cw.Topic)
+	if err != nil {
+		return errors.Wrapf(err,
+			"subscribing commandWebhook to %s topic", cw.Topic)
+	}
+
+	go func() {
+		for {
+			select {
+			case event := <-connectEvents:
+				var ev connect.Event
+				if err := connect.UnmarshalEvent(event.Message, &ev); err != nil {
+					fmt.Println(err)
+					continue
+				}
+
+				_, err := cw.HTTPClient.Post(cw.CallbackURL, contentType, bytes.NewBuffer(ev.Raw))
+				if err != nil {
+					fmt.Printf("error sending command response: %s\n", err)
+				}
+			}
+		}
+	}()
+
+	return nil
+}