feat: Add Swift Package Manager (SPM) support for iOS

[Mohammad-amleh]

Feature Request

CocoaPods will become permanently read-only on December 2, 2026 (see CocoaPods announcement).
Flutter has introduced Swift Package Manager as a replacement, available since Flutter 3.24+
via flutter config --enable-swift-package-manager.

When SPM is enabled, Flutter checks each plugin for a Package.swift file in its ios/ or
darwin/ directory. Plugins without Package.swift fall back to CocoaPods.

Currently, amplify_auth_cognito and amplify_secure_storage do not have Package.swift files,
so they are the only Amplify plugins that still require CocoaPods on iOS.

Additional context

• Flutter SPM tracking issue: ☂️ Add Swift Package Manager compatibility flutter/flutter#126005
• CocoaPods deprecation timeline: Read-only on December 2, 2026
• The native Amplify Swift SDK already supports SPM
  (https://github.com/aws-amplify/amplify-swift), so the underlying dependencies are ready.
• These Amplify Flutter plugins have no external CocoaPods trunk dependencies -- they only
  depend on Flutter itself, making the migration straightforward.

Environment

• Flutter: 3.38.9 (stable)
• amplify_flutter: ^2.6.0
• amplify_auth_cognito: ^2.6.0

[jvh-aws]

@Mohammad-amleh thanks a lot for creating this issue. We will investigate and come back to you.