fix(capacitor-ios): add root-level podspec for CocoaPods discovery

Capacitor CLI generates Podfile entries pointing to the plugin package
root, but the podspec was only in ios/. This caused pod install to fail
with "No podspec found" when using the plugin as a local dependency.

Author: j3k0

capacitor/CapacitorPluginPurchase.podspec

@@ -0,0 +1,18 @@
+require 'json'
+
+package = JSON.parse(File.read(File.join(__dir__, 'package.json')))
+
+Pod::Spec.new do |s|
+  s.name         = 'CapacitorPluginPurchase'
+  s.version      = package['version']
+  s.summary      = package['description']
+  s.license      = package['license']
+  s.homepage     = package['repository']['url']
+  s.author       = 'Jean-Christophe Hoelt'
+  s.source       = { :git => package['repository']['url'], :tag => s.version.to_s }
+  s.source_files = 'ios/Sources/PurchasePlugin/**/*.{swift,h,m}'
+  s.ios.deployment_target = '15.0'
+  s.swift_version = '5.9'
+  s.dependency 'Capacitor'
+  s.frameworks   = 'StoreKit'
+end