feat(connectivity_plus): Add Swift Package Manager support

[charafau]

ref: https://github.com/fluttercommunity/plus_plugins/issues/3152

Another spm support

removed obsolete objc code, moved plugin to swift only, which avoid mixing swift project

[charafau]

The example pubspec should not be changed.

Instead, use melos bootstrap as explained in the contributors document to setup the example project.

Thank you Miguel, will educate myself with melos :)