AP_Scripting: Support sending arbitrary mavlink messages #50852

Job	Run time
build (clang)	6m 42s
build (base)	6m 43s
autotest (sitltest-plane-tests1c)	5m 44s
autotest (sitltest-quadplane)	20m 24s
autotest (sitltest-plane-tests1a)	14m 8s
autotest (sitltest-plane-tests1b)	25m 25s
	1h 19m 6s

Provide feedback