Bridge wstring fields by converting between UTF-16 and UTF-8

[sloretz]

Feature request

Feature description

ROS 2 messages have a wstring field that can contain UTF-16 characters. The wstring design article (ros2/design#117) says when bridging a wstring field between ROS 2 and ROS 1 the bridge should attempt to convert UTF-16 to UTF-8 and pass the data as a ROS 1 string type. If the conversion fails then the message should be dropped. The same should be true in the other direction where the bridge should attempt to convert UTF-8 from ROS 1 to UTF-16.

Implementation considerations