Build for MacOS 10.14

[ahmad88me]

This PR addresses the issue: #434

Key Changes:

• The project now successfully builds on macOS 10.14.
• Note: While the code builds on macOS 10.14, the SignInWithApple functionality still requires macOS 10.15 due to Apple's platform restrictions.

Why is this change important?

Previously, even if the SignInWithAppleButton was only required for iOS, the build would still fail on macOS 10.14 due to missing APIs introduced in macOS 10.15. This meant that any project targeting macOS 10.14, even if only for non-sign-in functionality, would encounter build errors.

Purpose:

This change allows developers to conditionally include SignInWithAppleButton for iOS without breaking the build on macOS 10.14. For example, this code pattern will now work as expected:

if (Platform.isIOS) {
  SignInWithAppleButton(...)
}

By making the build compatible with macOS 10.14, we ensure that developers who don’t need macOS-specific sign-in functionality can still compile their apps, while the sign-in feature remains available for macOS 10.15 and above.

Testing:

I have tested this on macOS 10.14. Further testing on macOS 10.15 and higher is recommended to confirm that the SignInWithApple functionality works as expected on those versions.

[tp]

Thanks! This looks good, but since you're the only one ever asking for this in all the years we've had this plugin, I am not sure it'd be smart to merge this right now.

Especially since no-one of the other core contributors has access to an old macOS machine to keep ensuring compatibility (and further GitHub actions don't offer them anymore either, so we could not even compile it in CI as a minimal sanity check).

So for now I would appreciate if you just keep up the fork for anyone interested.

If we need a major update in the future (for whatever changes might come), we might want to re-evaluate. (Though maybe on the flip-side Flutter itself might've dropped support for this version in their then latest release…)