Add DSL transform() variant overload for accepting a bean name 

As discussed on thread for Issue #8625, there is a utility to have IntegrationFlows methods that accept a bean name, as is the case for 

```split(String beanName, @Nullable String methodName)``` 

The ```transform()``` method does not have such a bean-name overload.  It can be useful to have this in order to avoid the boilerplate injection of the bean via construction or bean method.
