Make r2s_gw an optional dependency #94

Job	Run time
determine-distros	3s
Generate Debian packages for ROS2 humble	9m 2s
Generate Debian packages for ROS2 jazzy	8m 29s
Generate Debian packages for ROS2 iron	8m 22s
Generate Debian packages for ROS2 kilted	9m 19s
Generate Debian packages for ROS2 rolling	8m 37s
Smoke test ROS2 iron	1m 26s
Smoke test ROS2 humble	1m 14s
Smoke test ROS2 rolling	1m 16s
Smoke test ROS2 kilted	1m 33s
Smoke test ROS2 jazzy	1m 16s
	50m 37s

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