feat: add support for formation key in app.json file

Still needs triggers for ps-can-scale and ps-scale-set.

Refs #4500

plugins/app-json/appjson.go

@@ -20,8 +20,9 @@ var (
 
 // AppJSON is a struct that represents an app.json file as understood by Dokku
 type AppJSON struct {
-	Cron    []CronCommand `json:"cron"`
-	Scripts struct {
+	Cron      []CronCommand        `json:"cron"`
+	Formation map[string]Formation `json:"formation"`
+	Scripts   struct {
 		Dokku struct {
 			Predeploy  string `json:"predeploy"`
 			Postdeploy string `json:"postdeploy"`
@@ -36,6 +37,10 @@ type CronCommand struct {
 	Schedule string `json:"schedule"`
 }
 
+type Formation struct {
+	Quantity int `json:"quantity"`
+}
+
 // GetAppjsonDirectory returns the directory containing a given app's extracted app.json file
 func GetAppjsonDirectory(appName string) string {
 	return filepath.Join(common.MustGetEnv("DOKKU_LIB_ROOT"), "data", "app-json", appName)