Add Message interface to rcljava_common

[Theosakamg]

For protect the design, I add Message interface

https://github.com/Theosakamg/ros2_java/tree/ros2_alpha7/rosidl_generator_java/src/main/java/org/ros2/internal/message
https://github.com/Theosakamg/ros2_java/blob/ros2_alpha7/rosidl_generator_java/resource/msg.java.template

[esteve]

Please do not use OSRF's copyright notice, it may mislead people to think this is an OSRF approved project. Also, I don't use the "@author" tag because it's hard to maintain, I prefer keeping a list of all the contributors (see https://github.com/esteve/ros2_java/blob/master/THANKS). The Apache Software Foundation removed the "@author" tag both for pragmatic and philosophical reasons, as it creates an imbalance in how communities build software.

In any case, the aim of this project is to mimic rclcpp, which does not use interfaces for messages. Could you open a pull request so we can discuss these changes? Thanks.