Add support for orthometric height

I use Mergin Maps with an external GPS device (connected by mock location through Trimble Mobile Manager).

It would be great if there was support in Mergin Maps to connect to a geoid-model, in order to measure in orthometric height (metres above sea level), rather than ellipsoidal height.

At the moment, I have to use workaround methods - either manipulate the z-value in postprocessing, or change the antenna height in the Mergin app so that it matches the difference between the geoid and the ellipsoid at the geographic place where I am currently doing field work.