feat(ekf2): EKF2_SENS_EN param, SensEnBit enum, MAVLink fusion command

Add EKF2_SENS_EN bitmask parameter (replaces EKF2_EN_BOOT) with
per-sensor enable bits. initFusionControl reads SENS_EN while disarmed.
handleSensorFusionCommand sets FusionSensor.enabled via
VEHICLE_CMD_ESTIMATOR_SENSOR_ENABLE. syncSensEnParam writes back to
param on disarm. Update EstimatorFusionControl.msg to bool
intended/active fields. Update VehicleCommand.msg FUSION_SOURCE enum.

msg/EstimatorFusionControl.msg

@@ -1,13 +1,23 @@
 uint64 timestamp                # time since system start (microseconds)
 
-int32 gps_intended              # effective EKF2_GPS_CTRL value (0 = disabled)
-int32 of_intended               # effective EKF2_OF_CTRL value (0 = disabled)
-int32 ev_intended               # effective EKF2_EV_CTRL value (0 = disabled)
-int32[4] agp_intended           # effective EKF2_AGPx_CTRL values (0 = disabled)
-int32 baro_intended             # effective EKF2_BARO_CTRL value (0 = disabled)
-int32 rng_intended              # effective EKF2_RNG_CTRL value (0 = disabled)
-int32 drag_intended             # effective EKF2_DRAG_CTRL value (0 = disabled)
-int32 mag_intended              # effective EKF2_MAG_TYPE value (5 = disabled/NONE)
-int32 imu_intended              # effective EKF2_IMU_CTRL value (0 = disabled)
+# sensor intended for fusion (enabled via EKF2_SENS_EN AND CTRL param != disabled)
+bool[2] gps_intended
+bool of_intended
+bool ev_intended
+bool[4] agp_intended
+bool baro_intended
+bool rng_intended
+bool mag_intended
+bool aspd_intended
+bool rngbcn_intended
 
-uint8 agp_active                # bitmask of AGP instances actively fusing (bit i = instance i)
+# whether the estimator is actively fusing data from each source
+bool[2] gps_active
+bool of_active
+bool ev_active
+bool[4] agp_active
+bool baro_active
+bool rng_active
+bool mag_active
+bool aspd_active
+bool rngbcn_active