Navigator: Add ability to wait for a position gate with executing the mission

This enables the navigator to wait for a specific gate coordinate to pass orthogonally to the current trajectory. This is particularly helpful for payload / camera handling in missions and avoids a dependency of payload handling on navigation waypoints.

src/modules/navigator/navigation.h

@@ -94,6 +94,7 @@ enum NAV_CMD {
 	NAV_CMD_FENCE_POLYGON_VERTEX_EXCLUSION = 5002,
 	NAV_CMD_FENCE_CIRCLE_INCLUSION = 5003,
 	NAV_CMD_FENCE_CIRCLE_EXCLUSION = 5004,
+	NAV_CMD_CONDITION_GATE = 4501,
 	NAV_CMD_INVALID = UINT16_MAX /* ensure that casting a large number results in a specific error */
 };