SITL: add simulated slung payload #38369

Job	Run time
build (base)	9m 11s
build-gcc-heli	7m 15s
build (clang)	8m 15s
autotest-heli (sitltest-heli)	10m 0s
autotest (sitltest-copter-tests1a)	7m 47s
autotest (sitltest-copter-tests1b)	9m 10s
autotest (sitltest-copter-tests1c)	11m 11s
autotest (sitltest-copter-tests1d)	8m 3s
autotest (sitltest-copter-tests1e)	8m 41s
autotest (sitltest-copter-tests2a)	9m 2s
autotest (sitltest-copter-tests2b)	28m 36s
	1h 57m 11s

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