Add support for iOS

[JlUgia]

• Add support for iOS with Apple Pay using a PlatformView to draw the button (@jamesblasco)
• Create both managed and raw buttons to support different integration use cases
• Separated iOS and Android into two different packages, such that users of the plugin can decide how to create their own plugins
• Moved the default platform channel implementation to the interface package
• Added common payment items and types

Closes #6.

[jamesblasco]

I would suggest to use a linter for this project. It will help a lot I think. There are some patterns that I haven't seen inside the Flutter Framework and I think there could be a better approach. To highlight the most important ones:

• Even if Dart allows dynamic values, the Flutter framework is strictly typed and I think it is a good practice to do so with external packages. Right now the payment buttons have all dynamic types, you can build something like the code below without any error during compilation, and lots of them during runtime

 GooglePayButton(
    paymentConfigurationAsset:10,
    paymentItems: 10,
    style: 10,
    type: 10,
    margin: 10,
    onPaymentResult: 10,
    loadingIndicator:10
  )

• Sometimes using nested expresions makes it hard to read and debug.
  https://dart.dev/guides/language/effective-dart/usage#consider-using--for-simple-members

https://github.com/google-pay/flutter-plugin-private/blob/72496a3527bee3a159183885e6ccaac39f0a1029/pay_platform_interface/lib/pay_channel.dart#L17-L26

@override
Future<Map<String, dynamic>> showPaymentSelector(
  Map<String, dynamic> profile,
  List<PaymentItem> items,
) async {
  final result = await channel.invokeMethod('showPaymentSelector', {
    'payment_profile': jsonEncode(profile),
    'payment_items': items.map((item) => item.toMap()).toList(),
  });
  return jsonDecode(result);
}

There is not good or wrong code about this but I think it improves it by following the style guidelines

[JlUgia]

Thank you for the review and comments @Arkangel12, @jamesblasco.
I've added changes and suggestions for all comments.
Feel free to review and resolve conversations as you see fit.

[Arkangel12]

minor comment. everything else looks good. I will approve it because it can be like it is, the change is after you.

[Arkangel12]

this is kind of a hard this to do, naming stuff, haha, but I will something related to the PayButton class maybe something like showChild.

[JlUgia]

All comments addressed.
Merging within the next few hours if there are no objections.

[domesticmouse]

I'm curious what you need from flutter/foundation.dart

[JlUgia]

To use the defaultTargetPlatform. See pay_button.dart and pay.dart

[domesticmouse]

Ahh right. Carry on =)