AP_GPS: Implement full covariance matrix on Septentrio

@Ryanf55 fair enough. For future reference it's here: https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_SBF.cpp#L494 and the struct is here: https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_SBF.h#L202

_Originally posted by @WickedShell in https://github.com/ArduPilot/ardupilot/issues/23259#issuecomment-1496684567_

This ticket is to add support for the Septentrio driver for full covariance since it's provided by the receiver. Similar tickets can be done for each vendor that supports it.

Start by reading libraries/AP_GPS/AP_GPS_SBF.cpp around line 494 and the covariance struct in libraries/AP_GPS/AP_GPS_SBF.h around line 202. Trace how the Septentrio receiver data reaches that struct, then add full covariance support using the covariance provided by the receiver; done means the driver exposes the complete matrix.

Written by the indexing model from the issue text.