Add customization points (by using 'entry_points') for including thir…

[cwecht]

…d party tracepoints in graph.

It is now possible to create extensions for ros2_profile.

• With the extension point 'ros2profile.custom_conversion_functions' thirdparty packages can define custom conversion function for parsing new (otherwise unsupported) tracepoints.
• With the extension point 'ros2profile.extend_graph' thirdparty packages can define extension which may incorporate new (custom) tracepoints into the graph.

The setup.py of an extentsion package could look like this: setup.py.txt