MM-21898 - Part 1: Generate and use an interface instead of *A… (#13840)

* Generate and use an interface instead of *App

app/command_echo.go

@@ -77,7 +77,7 @@ func (me *EchoProvider) DoCommand(a *App, args *model.CommandArgs, message strin
 	}
 
 	echoSem <- true
-	a.Srv.Go(func() {
+	a.Srv().Go(func() {
 		defer func() { <-echoSem }()
 		post := &model.Post{}
 		post.ChannelId = args.ChannelId