text: Title: [Trigger Actions UI] Allow async alert type registration from plugins
  Body: **Describe the feature:**
  For each alert type to work in alerting UI, each plugin has to register an alert type in start contract, that ends up adding few additional bytes into plugin page load bundle sizes, for example for uptime plugin
  
  ` plugins.triggersActionsUi.alertTypeRegistry.register(alertInitializer);
  `
  
  Plugin ends up importing all alert types defined and those becomes part of page load bundle, it's not a significant chunk, it contains few constants, strings etc, but if we combine for example 20-30 alert types , it might be worth a try to allow plugins to register a promise, and then when alerting ui is loaded, it loads those alert types async. 
  
  `TypeRegisrty,register` function will have to be modified to be acceptable for this proposal. 
  
  **Describe a specific use case for the feature:**
  Improving page load bundle sizes for each plugin, hence improving kibana performance, less code to load and parse. 
  