To allow us to determine the type that Spring Integration’s
GatewayProxyFactoryBean will create, the bean definition created by
MessagingGatewayRegistrar needs to set the factoryBeanObjectType
attribute. The current implementation of BeanTypeRegistry requires the
attribute’s value to be a Class, however this would require Spring
Integration’s namespace handler to load the class and class loading
should be avoided in namespace handlers.

This commit updates BeanTypeRegistry so that it supports both Class and
String values for the factoryBeanObjectType. If the value is a String
it will interpret it as a class name and attempt to load it.

See gh-2811