Introduce method-invoking `handle`

* Add generic argument to `GenericEndpointSpec`.
Since `EndpointSpec#get()` returns `Tuple` with `EndpointFactoryBean` and `MessageHandler` objects
it is useful to get deal with specific generic for further `MessageHandler` configuration within `EndpointConfigurer` lambda
* Change DSL-methods to get deal deal with those generics.
* Add `FileWritingMessageHandler` test to demonstrate it.
* Fix bug around double `MessageHandler` bean registration from `DslIntegrationConfigurationInitializer`, when handler is a reference
to existing bean.