Feature request - add support for GameSave

**Is your feature request related to a problem? Please describe.**
I'd like to use GameSave to synchronize save games across devices and game versions. The abstraction is much closer to how cloud sync works for other platforms and I'd like to take advantage of it in a Unity project.

**Describe the solution you'd like**
An officially supported Unity plugin for the GameSave feature.
https://developer.apple.com/documentation/GameSave

**Describe alternatives you've considered**
I could try to implement it myself. But I'm not an expert in this area.

The issue names no repository files, tests, or entry points. Start by reading Apple's GameSave documentation and examining the existing Unity plugin structure to determine the integration surface. Done should mean an officially supported Unity plugin that synchronizes save games across devices and game versions.

Written by the indexing model from the issue text.