Support for Connext 6

[dirk-thomas]

Some preliminary work regarding Connext 6 support has landed in #371, ros2/rosidl_typesupport_connext#37, and #374 to allow adding Connext 6 support at any time without breaking anything. The patch file connext_static_serialized_dataPlugin.cxx.patch which contains the heavy lifting and still needs to be updated for Connext 6 though.

@Karsten1987 @neil-rti I will leave this up to either of you (or anyone else) to contribute the missing bits in a new pull request. Until then ROS 2 will be limited to only support Connext 5.

With the conditional logic in place such a change can land any time - it just won't change the Connext version we are building against in already released distributions though - since that is specified in REP 2000 and locked before a release.