ExternalAHRS: support variances in backends

[tridge]

simplifies backends, allows GCS to see variances in EKF message, and allows for copter EKF failsafe to work
broken out of #24747

[Georacer]

Left some comments.

Has there been any kind of testing?

[amilcarlucas]

Binary Name      Text [B]         Data [B]     BSS (B)      Total Flash Change [B] (%)      Flash Free After PR (B)
---------------  ---------------  -----------  -----------  ----------------------------  -------------------------
arducopter-heli  -136 (-0.0075%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0075%)                                  142992
antennatracker   -136 (-0.0101%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0101%)                                  615516
arducopter       -136 (-0.0075%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0075%)                                  142464
ardurover        -136 (-0.0082%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0081%)                                  296648
arduplane        -136 (-0.0075%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0075%)                                  145904
blimp            -136 (-0.0099%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0099%)                                  593488
ardusub          -136 (-0.0084%)  0 (0.0000%)  0 (0.0000%)  -136 (-0.0084%)                                  352356

[tridge]

@Georacer I had stuffed up the scaling! Fixed up now