Add Apple Pay as the first payment mechanism on iOS

[JlUgia]

This issue captures the main characteristics of adding support for payments on iOS through Apple Pay. This is intended as a design foundation (and a discussion if applicable).

Plugin principles:

• It supports multiple payment options within a single platform (if available).
• Idiomatic, useful and unsurprising (it uses strict code style guidelines for the languages used).
• Testable automatically, and released shortly after changes are added and stabilized.
• Built on top of modern and stable technology (Flutter 2, Dart 2.12, Swift, Kotlin, federation).
• Simplicity over flexibility (with alternative integration paths for further customization).
• Open and supporting as many configurations as possible (PSPs, native features).

Apple Pay integration breakdown:

• Use Swift (and follows Swift's API design guidelines).
• Define a JSON-based spec to allow for the creation of exchangeable files that can be loaded once from any source, locally or remotely (use the W3C payment standard spec for Apple Pay as a source for inspiration, except for the display items, which will be added at runtime – see Add support for display items to calculate the total price #6).
• Create a raw button that is conceptually separate from the payment logic.

Discussions / challenges

• To maximize the flexibility of mixing and matching federated plugins, the intention is to separate Android and iOS into separate folders/sub-plugins.
• If using the raw assets from Apple Pay to draw the button on the Flutter end is not an option, use a PlatformView to surface the programatic button all the way to the Flutter tree.
• Verify the required steps to use the token returned against public endpoints for different PSPs.
• Supporting dynamic price updates will likely require an additional call to the method channel. Consider deferring this functionality to later versions of the library.

Please do feel free to add to this.
Let's use this overarching foundation to plan the addition of Apple Pay, and potential breakdown of the effort.