[iOS] Add Swift Package Manager (SPM) support

[paoloartasensi]

Hi maintainers,

The plugin flutter_foreground_task
currently triggers a warning when building for iOS with recent Flutter versions:

The following plugins do not support Swift Package Manager for ios: flutter_foreground_task. This will become an error in a future version of Flutter. Please contact the plugin maintainers to request Swift Package Manager adoption.

Are there any plans to migrate the iOS implementation to support SPM? This will soon be required for all Flutter iOS builds, and CocoaPods support is being phased out.

Thank you for your work on this package!