msg/vehicle_odometry.msg: simplify covariance handling and update all usage (#19966)

- replace float32[21] URT covariances with smaller dedicated position/velocity/orientation variances (the crossterms are unused, awkward, and relatively costly)
 - these are easier to casually inspect and more representative of what's actually being used currently and reduces the size of vehicle_odometry_s quite a bit
 - ekf2: add new helper to get roll/pitch/yaw covariances
 - mavlink: receiver ODOMETRY handle more frame types for both pose (MAV_FRAME_LOCAL_NED, MAV_FRAME_LOCAL_ENU, MAV_FRAME_LOCAL_FRD, MAV_FRAME_LOCAL_FLU) and velocity (MAV_FRAME_LOCAL_NED, MAV_FRAME_LOCAL_ENU, MAV_FRAME_LOCAL_FRD, MAV_FRAME_LOCAL_FLU, MAV_FRAME_BODY_FRD)
 - mavlink: delete unused ATT_POS_MOCAP stream (this is just a passthrough)

Co-authored-by: Mathieu Bresciani <brescianimathieu@gmail.com>

src/modules/ekf2/EKF/common.h

@@ -207,7 +207,7 @@ struct extVisionSample {
 	Vector3f    vel{};         ///< FRD velocity in reference frame defined in vel_frame variable (m/sec) - Z must be aligned with down axis
 	Quatf       quat{};        ///< quaternion defining rotation from body to earth frame
 	Vector3f    posVar{};      ///< XYZ position variances (m**2)
-	Matrix3f    velCov{};      ///< XYZ velocity covariances ((m/sec)**2)
+	Vector3f    velVar{};      ///< XYZ velocity variances ((m/sec)**2)
 	float       angVar{};      ///< angular heading variance (rad**2)
 	VelocityFrame vel_frame = VelocityFrame::BODY_FRAME_FRD;
 	uint8_t     reset_counter{};