build(packaging): add PX4 SITL .deb packages

Add cmake/cpack infrastructure for building .deb packages from
px4_sitl_sih and px4_sitl_default targets. Includes install rules,
package scripts, Gazebo wrapper, and CI workflow.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

Tools/packaging/postrm

@@ -0,0 +1,6 @@
+#!/bin/sh
+set -e
+if [ "$1" = "remove" ] || [ "$1" = "purge" ]; then
+    rm -f /usr/bin/px4-gazebo
+fi
+exit 0