Add Swift Package Manager support for iOS plugins

[MohamedGawdat]

Summary

• add Flutter-compatible Swift Package Manager manifests for zohodesk_portal_apikit, zohodesk_portal_configuration, and zohodesk_portal_submit_ticket
• pin the official native Swift packages to 4.6.8, matching the existing podspecs
• move plugin sources into the shared SPM/CocoaPods source layout and update the podspecs
• remove cross-plugin Swift imports by keeping the small JSON decoding helpers private to each target
• allow Dart 3 so the packages can be validated with Flutter 3.44
• update stale platform-interface test mocks

Fixes #39.

Validation

• built an unsigned pure-SPM iOS app with Flutter 3.44.7
• confirmed Flutter reports all included iOS plugins as Swift packages with no CocoaPods fallback
• passed the test suites for all three changed Flutter packages
• verified all updated podspecs parse with CocoaPods 1.17.0

The build used all three plugins together, plus SalesIQ Mobilisten, to exercise the combined native dependency graph.