AP_NavEKF3: fixes in VelPos fusion

[rishabsingh3003]

This PR attempts the following:

1. Slight cleanup of VelPosFusion(). Instead of using bool fusePosData and bool fuseVelData, I replace it with a enum to get more information about the source of the sensor being fused. I have also tried to get rid of some of the global variables which don't need to be global.
2. Allow ExtNav and GPS to work without Vel Z being fused. Currently, if you have VELXY defined in the Sources, the Z component also gets fused irrespective (bug fix). Replaces: AP_NavEKF3: Allow extnav to work without vertical velocity #29213
3. Use the correct External Nav variances when both GPS and Ext Nav are being used (bug fix) Closes: External navigation measurement noise is overwritten by GPS reported accuracy in EKF #24020.