feat: add swift package manager support

[fadi-george]

Description

One Line Summary

Adds swift package manager support for the flutter sdk package.

Details

Closes #971
Follows https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors#how-to-turn-on-swift-package-manager guide for setting up spm for flutter plugins.

• update projects to use spm which should still be compatible for cocoapods projects

Motivation

• cocoapods support is deprecated in favor swift package manager support

Testing

Manual testing

Test step 20 for https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors#how-to-turn-on-swift-package-manager
where I disabled spm config and did flutter run in the example project

Then I re-enabled spm config and did the same thing and it still worked.

Pods:

Show pod install

SPM:

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is 

[nan-li]

1. Duplicate source files in ios/Classes/ and ios/onesignal_flutter/Sources/onesignal_flutter/. Step 8 and 9 of guide notes to remove old directory:

Move all files from ios/Classes to ios/plugin_name/Sources/plugin_name.
The ios/Assets, ios/Resources, and ios/Classes directories should now be empty and can be deleted.

2. Info on documentation steps needed? This is the existing Flutter SDK setup: https://documentation.onesignal.com/docs/en/flutter-sdk-setup. Can you add any notes on how we would update the docs?

[fadi-george]

Seems like for NSE and live activity widget they both need to manually add OneSignalXCFramework to "Frameworks and Libraries" section