feat: add support for non-web healthchecks via app.json

This change converts existing CHECKS files into a healthchecks key that is understood by 'docker-container-healthchecker'. This tool supports a number of different types of container healthchecks - command, http, uptime - and can perform healthchecks against non-web processes.

The use of the old CHECKS file is now deprecated, and will be removed in the next minor version. Users can use the 'docker-container-healthchecker' to convert existing CHECKS files to the new format automatically.

Closes #2760

plugins/app-json/src/triggers/triggers.go

@@ -22,6 +22,9 @@ func main() {
 		appName := flag.Arg(0)
 		processType := flag.Arg(1)
 		err = appjson.TriggerAppJSONProcessDeployParallelism(appName, processType)
+	case "app-json-get-content":
+		appName := flag.Arg(0)
+		err = appjson.TriggerAppJSONGetContent(appName)
 	case "core-post-deploy":
 		appName := flag.Arg(0)
 		err = appjson.TriggerCorePostDeploy(appName)