Add SwiftPM support

[dh0er]

Summary

• Adds Swift Package Manager support for auto_updater_macos while keeping CocoaPods compatibility.
• Moves macOS Swift sources into Flutter's SwiftPM package layout and adds a Sparkle >=2.9.2 <3.0 dependency.
• Refactors macOS Sparkle tool discovery to support AUTO_UPDATER_SPARKLE_BIN, CocoaPods, and SwiftPM artifact paths, with tests.
• Updates docs, changelogs, CI Flutter version, and example macOS metadata.

Minimum Versions

• Dart: raised from >=3.0.0 to >=3.11.0
• Flutter: raised from >=3.3.0 to >=3.41.0
• macOS deployment target: raised to 10.15
• Sparkle: raised to >=2.9.2 <3.0 for CocoaPods and SwiftPM

Notes

• window_manager and screen_retriever_macos warnings come only from the example app dependencies, not from auto_updater itself.
• The macOS CI workflow clears SwiftPM binary artifact cache before building to avoid stale hosted-runner cache failures.

Validation

• melos run format-check
• melos run analyze
• melos run test --no-select
• flutter build macos --release with SwiftPM enabled
• flutter clean && flutter build macos --release with SwiftPM disabled/CocoaPods
• GitHub Actions build-macos passes after cache cleanup