[flight plan] add on_enter and on_exit functions for nav block (#3385)

The functions passed to the attributes 'on_enter' and 'on_exit' will be
called once every time the flight plan state machine enters or leaves a
block, even if the block change is requested by the operator or if it is
caused by an exception.
It allows to properly activate and then disable services related to a
specific block.
Another good side effect is that the block and stage index are now
private and can't be modified by external modules. Old variables
nav_block and nav_stage can still be used as read-only.

Based on the work from:
Baptiste Pollien, Christophe Garion, Gautier Hattenberger, Pierre Roux, Xavier Thirioux.
A Verified UAV Flight Plan Generator. 2023 IEEE/ACM 11th International Conference on Formal Methods in Software Engineering (FormaliSE),
May 2023, Melbourne, Australia. pp.130-140, 10.1109/FormaliSE58978.2023.00021

sw/airborne/firmwares/fixedwing/nav.c

@@ -531,8 +531,7 @@ static void send_survey(struct transport_tx *trans, struct link_device *dev)
  */
 void nav_init(void)
 {
-  nav_block = 0;
-  nav_stage = 0;
+  common_flight_plan_init();
   ground_alt = GROUND_ALT;
   nav_glide_pitch_trim = NAV_GLIDE_PITCH_TRIM;
   nav_radius = DEFAULT_CIRCLE_RADIUS;

sw/airborne/firmwares/rotorcraft/navigation.c

@@ -70,9 +70,7 @@ static inline void nav_set_altitude(void);
 void nav_init(void)
 {
   waypoints_init();
-
-  nav_block = 0;
-  nav_stage = 0;
+  common_flight_plan_init();
 
   nav.horizontal_mode = NAV_HORIZONTAL_MODE_WAYPOINT;
   nav.vertical_mode = NAV_VERTICAL_MODE_ALT;

sw/airborne/firmwares/rover/navigation.c

@@ -79,9 +79,7 @@ static void send_wp_moved(struct transport_tx *trans, struct link_device *dev)
 void nav_init(void)
 {
   waypoints_init();
-
-  nav_block = 0;
-  nav_stage = 0;
+  common_flight_plan_init();
 
   nav.mode = NAV_MODE_WAYPOINT;