feat(pay_ios): Add Swift Package Manager support

pay_ios/ios/pay_ios.podspec

@@ -10,14 +10,13 @@ Pod::Spec.new do |s|
                        DESC
   s.homepage         = 'https://github.com/google-pay/flutter-plugin'
   s.license          = { :file => '../LICENSE' }
-  s.author           = { 'Google Pay Developer Relations' => 'payments-devrel-flutter@googlegroups.com
-  ' }
+  s.author           = { 'Google Pay Developer Relations' => 'payments-devrel-flutter@googlegroups.com' }
   s.source           = { :http => 'https://github.com/google-pay/flutter-plugin' }
-  s.source_files = 'Classes/**/*'
+  s.source_files = 'pay_ios/Sources/pay_ios/**/*'
   s.dependency 'Flutter'
-  s.platform = :ios, '8.0'
+  s.platform = :ios, '13.0'
   s.frameworks  = 'PassKit'
   # Flutter.framework does not contain a i386 slice.
   s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386' }
   s.swift_version = '5.0'
-end
+end

pay_ios/ios/pay_ios/Package.swift

@@ -0,0 +1,19 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+let package = Package(
+    name: "pay_ios",
+    platforms: [
+        .iOS("13.0")
+    ],
+    products: [
+        .library(name: "pay-ios", targets: ["pay_ios"])
+    ],
+    dependencies: [],
+    targets: [
+        .target(
+            name: "pay_ios",
+            dependencies: []
+        )
+    ]
+)