Merge PR #11253: HITL/SITL X-Plane improvements

- MSP_SIMULATOR v2/v3 protocol: refactor into readMspSimulatorValues()
  and mspProcessSimulatorCommand(); v3 uses 2-byte flags field
- New HITL flags: HITL_RANGEFINDER, HITL_CURRENT_SENSOR, HITL_SIM_RC_INPUT,
  HITL_FAILSAFE_TRIGGERED, HITL_SITL_MODE
- Fix GPS velocity: exclude HITL mode from validVelNE path (same as SITL)
- battery.c: inject simulated current when HITL_CURRENT_SENSOR active
- displayport_msp_osd.c: enable OSD vtxActive in SITL mode
- runtime_config: add SIMULATOR_MSP_VERSION_3 and new HITL flag bits
- common.h: promote RX_SIM and RANGEFINDER_FAKE out of SITL-only target.h
- xplane.h: add stdbool.h include and simXPlaneClose() declaration
- xplane.c: INAV-XITL plugin with DataRef state machine; add atomic
  thread safety and simXPlaneClose() from maintenance-9.x

Author: sensei-hacker

docs/SITL/SITL.md

@@ -18,7 +18,7 @@ INAV SITL communicates for sensor data and control directly with the correspondi
 
 AS SITL is still an inav software, but running on PC, it is possible to use HITL interface for communication.
 
-INAV-X-Plane-HITL plugin https://github.com/RomanLut/INAV-X-Plane-HITL can be used with SITL.
+[INAV-X-Plane-HITL](https://github.com/RomanLut/INAV-X-Plane-HITL) or [INAV-X-Plane-XITL](https://github.com/Scavanger/INAV-X-Plane-XITL) plugin can be used with SITL.
 
 ## Sensors
 The following sensors are emulated:

docs/SITL/X-Plane.md

@@ -7,6 +7,9 @@ X-Plane is not a model flight simulator, but is based on real world data and is
 ## Aircraft
 It is recommended to use the "AR Wing" of the INAV HITL project: https://github.com/RomanLut/INAV-X-Plane-HITL
 
+## INAV Plugin
+For advanced SITL featues (like OSD, virtual RX, simulated hardware failures, power train simulation) you can use the [INAV-X-Plane-XITL plugin](https://github.com/Scavanger/INAV-X-Plane-XITL)
+
 ## General settings
 In Settings / Network select "Accept incoming connections".
 The port can be found under "UDP PORTS", "Port we receive on". If no connection is established, the port can be changed.

docs/development/Hardware In The Loop (HITL) plugin for X-Plane.md

@@ -8,4 +8,10 @@
 
 https://github.com/RomanLut/INAV-X-Plane-HITL
 
+or for latest HITL features (INAV >= 9.0)
+
+ **INAV-X-Plane-XITL** 
+ 
+ https://github.com/Scavanger/INAV-X-Plane-XITL
+
 HITL technique can be used to test features during development. Please check page above for installation instructions.