Introduce application properties to define Push transport and mode

[glebfox]

In CUBA we have cuba.web.pushLongPolling that enables switching to long polling instead of WebSocket for server push implementation.

Currently, there is no alternative for Jmix and it's hard to change Push transport

[glebfox]

To change Push transport the custom AppUI class must be created. To do this:

1. Create a custom AppUI class

@Push(transport = Transport.LONG_POLLING)
public class CustomAppUI extends AppUI {
}

2. Register this bean

   @Bean
   public AppUI customAppUI() {
   return new CustomAppUI();
   }

3. Exclude built-in AppUI class in application.properties:

jmix.core.exclude-beans = appUI