Build fails if you don't have fastrtps

[ivanpauno]

The building of rosbag2_converter_default_plugins is failing not quietly when you don't have rmw_fastrtps_cpp. I think that's not ok, you should be able to build ros2 without it (e.g.: only with opensplice or connext).

Failing CI builds with only opensplice (from comment):

• Linux
• Linux-aarch64
• macOS
• Windows

Further info:

The problem is here:

rosbag2/rosbag2_converter_default_plugins/CMakeLists.txt

Lines 53 to 57 in 2ad74a4

	if(BUILD_TESTING)
	find_package(ament_cmake_gmock REQUIRED)
	find_package(ament_lint_auto REQUIRED)
	find_package(rcutils REQUIRED)
	find_package(rmw_fastrtps_cpp REQUIRED)

The build is only failing when BUILD_TESTING=1, but that's usual.

[dirk-thomas]

@Karsten1987 can you please address this since it makes building without FastRTPS more effort.