Support custom geoid models

[Withalion]

Currently there has been work done (#4234) to use orthometric heights by default. By default we use EGM96 geoid model bundled with the application. This model is precise enough for common work. However some high accuracy projects require setting a custom geoid model as mentioned here.

The idea is that we let admins set up custom geoid models in the QGIS plugin (MerginMaps/qgis-plugin#841), which will have to be packed with the project. Then the application uses this model for transformations.

[IvaKuklica]

Hi @Withalion

Testing of bluetooth (external GNSS) provider:

Application (+ app version, build, operating system):
Android 15/ Build 739951

Project: test_external_receiver_world
WGS 84_EGM2008 geoid

SUMMARY:
The difference between a feature recorded in Emlid Flow and Mergin Maps applications is on average 0.007 m.
ISSUE:
The warning message containing information regarding proj file/restarting of app is not being displayed.

Project: test_external_receiver_martinique
IGN 1987 (MARTINIQUE) geoid

ISSUE:
Geoid separation and Altitude display a values and it should be N/A.
The warning message containing information regarding proj file/restarting of app is not being displayed.