Add basic concepts for event bus, and implement in request and model

src/index.ts

@@ -1,6 +1,11 @@
 export * from './util'
 export * from './di'
 
+export * from './event/types'
+export * from './event/Event'
+export * from './event/EventBus'
+export * from './event/PropagatingEventBus'
+
 export * from './service/Logging'
 
 export * from './lifecycle/RunLevelErrorHandler'