Use AP_GPS_FixType in AP_ExternalAHRS

[peterbarker]

... rather than just a uint8_t.

This is supposed to be the conversion from externalahrs concept of fix type into ArduPilot's, so use an ArduPilot type for it.

Replaces https://github.com/ArduPilot/ardupilot/pull/23015/files

[peterbarker]

This only affects boards which include ExternalAHRS.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            0      *           -8      -8                0      -8     -8
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     0      *           -8      -8                0      0      -8
MatekF405                           *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           *      *           *       *                 *      *      *
skyviper-v2450                                         *                                       

relying on simulators to pick problems up here.