feat: add ability to specify a concurrency policy for crons in the app.json file

plugins/app-json/appjson.go

@@ -60,6 +60,9 @@ type CronTask struct {
 
 	// Schedule is the cron schedule to execute the command on
 	Schedule string `json:"schedule"`
+
+	// ConcurrencyPolicy is the concurrency policy for the cron command
+	ConcurrencyPolicy string `json:"concurrency_policy"`
 }
 
 // Formation is a struct that represents the scale for a process from an app.json file