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?