configurable QoS

[BrettRD]

QoS incompatibility is a thing, the default middleware defaults everything to reliable, which cannot subscribe to lesser QoS publishers.
The src nodes should set their default QoS as low as possible, and the sink nodes high as possible.
All QoS settings should be configurable.

[BrettRD]

The QoS is using a convention established in gazebo plugins that shouldn't cause any incompatibility headaches, but may have a buffers too short for some applications.
The elements should have a gstreamer property to set the QoS reliability and queue depth at least.