Break up `Desktop` and `Apple` submodule into specific platforms

[bruno-garcia]

Today our support for Windows are Linux lives on Desktop. And iOS and macOS on Apple

That was done this way since we mainly point to an underlying Sentry SDK (sentry-native and sentry-cocoa respectively).
But Unreal itself has platform specific subsystems like macOS, Linux, iOS, etc.

There are several advantages with breaking up the code in the same structure as compared with #ifdef throughout the files.

[tustanivsky]

Closing this since #745 which improves platform-specific code separation was already merged into main.