feat(ios): add Swift Package Manager support

[TeddyYeung]

Why

Flutter 3.44+ makes Swift Package Manager (SPM) the default for iOS builds. CocoaPods trunk will be read-only from December 2, 2026. Plugins without SPM support will require users to pass `--disable-swift-package-manager` on every build.

This PR adds iOS SPM support to `appsflyer_sdk` so it works seamlessly with modern Flutter toolchains.

What changed

• Added `ios/appsflyer_sdk/Package.swift` — SPM manifest (swift-tools-version 5.9, iOS 12.0)
• Added `ios/appsflyer_sdk/Sources/appsflyer_sdk/` — ObjC SPM target (mirrors Classes/ ObjC files)
• Updated `CHANGELOG.md`
• Updated `.gitignore` — added `.build/` and `.swiftpm/`

Architecture note

• The plugin is ObjC-only (no Swift files in Classes/), so SPM supports it as a single ObjC target.
• `AppsFlyerFramework` 6.18.0 is declared as an SPM binary dependency (already has SPM support via xcframework).
• CocoaPods path (`ios/Classes/`) is unchanged — full backward compatibility.

Testing

• ✅ `flutter build ios --no-codesign` with SPM enabled — Build succeeded
• ✅ `flutter build ios --no-codesign` with CocoaPods (SPM disabled) — Build succeeded

Tested with Flutter 3.35.7 on macOS.

Breaking Changes

None. CocoaPods behavior is unchanged.

References

• Flutter SPM guide: https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors
• AppsFlyerFramework SPM: https://github.com/AppsFlyerSDK/AppsFlyerFramework

[yanashenyang]

The following plugins do not support Swift Package Manager for ios:

• appsflyer_sdk
  This will become an error in a future version of Flutter. Please contact the plugin maintainers to request Swift Package Manager adoption.

[yanashenyang]

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