ROS topic transport improvements

[meyerj]

Follow-up on https://github.com/orocos/rtt_ros2_integration/pull/10:

• [ ] A ROS service that can be loaded into a component which provides operations to connect ports to ROS publishers and subscribers with custom QoS settings, bypassing the createStream() API and without the limitations of the ConnPolicy struct.
• [ ] Add a more complex test case with at least two processes that communicate via ROS topics.
• [ ] Move common CMake code from rtt_ros2_idl-generate_typekit.cmake and rtt_ros2_topics-generate_ros_transport.cmake to another helper script to share it, e.g. for retrieving a list message, service and action types provided by a given package. At the moment the second script is a simplified and adapted copy of the first.