Add support to Swift Package Manager

[ziadsarour]

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Is there an existing issue?

• [x] I have searched existing issues

Use case

Add support to Swift Package Manager as recommended by Flutter (package objectbox_flutter_libs)

Proposed solution

https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors

[greenrobot-team]

Thanks! We are also aware. But we likely won't work on this in the near future as the ObjectBox Swift (which the Dart library uses internally to support Apple devices) support for Swift Package Manager is still experimental and the Flutter docs explicitly note:

Flutter's support of Swift Package Manager is under development. Swift Package Manager support is off by default. Flutter continues to support CocoaPods.