make producer / consumer plugins in a shared library and load them with pluginlib

[dirk-thomas]

Simple example using pluginlib (https://github.com/ros/pluginlib/tree/ros2) and indirectly class loader (https://github.com/ros/class_loader/tree/ros2).

Before this can be considered it must be possible to build a shared library which only depends on rclcpp but not a specific rmw implementation. Currently rclcpp depends directly on the headers of a specific implementation:

• https://github.com/ros2/rclcpp/blob/65300405bef94cc391253083f7140041c69c737f/rclcpp/include/rclcpp/node.hpp#L27
• https://github.com/ros2/rclcpp/blob/65300405bef94cc391253083f7140041c69c737f/rclcpp/include/rclcpp/node_impl.hpp#L30-L31

[dirk-thomas]

This has been implemented in #84.