feat(pay_ios): Add Swift Package Manager support

[Robert-SD]

• feat(pay_ios): add Swift Package Manager support

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Describe the changes proposed

• This PR introduces Swift Package Manager (SPM) support for the pay_ios component while keeping CocoaPods support via the existing podspec.
• It also drops support for iOS 8-12 for apps consuming this plugin version. The latest Flutter version already targets iOS 13 as the minimum version. Would that breaking change be okay?

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[johnpryan]

@Robert-SD could you please sign the CLA? Thank you!

[Robert-SD]

Hey @johnpryan, just did that! Thanks for the reminder. I changed the commit history and removed the bot from it.
Regarding the PR, would changing the minimum iOS version to 13 be okay?

[Robert-SD]

I saw in the blog post that the FlutterFramework should be added as a dependency. Will add that soon.

[Robert-SD]

Adopting step 10 of the migration guide (FlutterFramework dependency) would force a minimum-Flutter bump to 3.41, which is a breaking change for users still on 3.16–3.40:

environment:
  sdk: ^3.11.0
  flutter: ">=3.41.0"

Forcing the bump would block those users from upgrading to the SPM-enabled version of the plugin at all. They'd be stuck on the last pre-SPM release.

I'm not exactly sure if bumping is the right approach in this PR. We could do it later.
How would you proceed?

[JlUgia]

I suggest we release a major for this.

[JlUgia]

Thanks for the proposal