Provide a conventional way to extend XD Container configuration

Provide an easy way for users to add beans (e.g., Gemfire cache configuration) or modify default XD configuration such as serializers, and message converters. A simple approach is to add a well known resource selector such as classpath*:META-INF/spring/xd/extensions or include this path in an extensible @Configuration base class. In addition, we should adopt conventional names for beans that are meant to be extended, e.g. use an xd. prefix.