Fix DllNotFoundException on unsupported Unity target platforms

[Habetdin]

When Apple Unity Plug-Ins are loaded in Unity Editor with target platform that is not supported by the Plug-Ins, Editor produces errors like these:

DllNotFoundException: AppleCoreNativeMac assembly:<unknown assembly> type:<unknown type> member:(null)
DllNotFoundException: GameKitWrapper assembly:<unknown assembly> type:<unknown type> member:(null)

This issue could be fixed by adding constraints like UNITY_IOS || UNITY_STANDALONE_OSX || UNITY_TVOS || UNITY_VISIONOS (that's taken from the original code) to the assemblies.

Replaces: #42

[Habetdin]

@AdamSzApple are PRs @ GitHub being monitored?

Or just the ones sent via Apple Feedback Assistant?

[pabloleban]

Hello @AdamSzApple

Many users (including me) are getting this issue, so it would be great if this PR can be merged so it's fixed to all of us and we don't have to manually change the code everytime we want to build the new version.

[JVimes]

@Habetdin Did this ever make it into the product? I see it's closed.

[Habetdin]

@JVimes it seems like Apple developers don't check PRs here, so I just closed this one. You can add constraints like UNITY_IOS to the assembly definitions of Apple Unity Plugins you have in the project manually.

P.S.: I've switched to 3rd-party Sign in with Apple plugin that doesn't rely on this repo — that fits all my needs 😅