AP_Scripting: add handle_external_position_estimate binding #4204

Job	Run time
build (base)	8m 7s
build (clang)	9m 46s
autotest (sitltest-rover)	17m 44s
autotest (sitltest-sailboat)	2m 37s
autotest (sitltest-balancebot)	7m 39s
	45m 53s

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