Add Apple services for macOS

[PoqXert]

Describe the project you are working on:
Cross-platform Game

Describe the problem or limitation you are having in your project:
The game use Apple GameCenter and purchases. It available on iOS and macOS, but Godot implements for iOS only.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Move implementation (GameCenter, purchases etc.). from core to module and add macOS.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
Same as now, but on two platforms.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
No.

Is there a reason why this should be core and not an add-on in the asset library?:
It in core.

[PoqXert]

Services moved from the core to plugins for iOS (godotengine/godot#41340, godotengine/godot#45336).
Now it remains to implement services as a GDNative for macOS.