Add in-app purchase methods

client/platforms/ios/StoreKitController.h

@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef STOREKITCONTROLLER_H
+#define STOREKITCONTROLLER_H
+
+#import <Foundation/Foundation.h>
+
+@interface StoreKitController : NSObject
+
++ (instancetype)sharedInstance;
+
+- (void)purchaseProduct:(NSString *)productIdentifier
+             completion:(void (^)(BOOL success, NSError *_Nullable error))completion;
+
+- (void)restorePurchasesWithCompletion:(void (^)(BOOL success, NSError *_Nullable error))completion;
+
+@end
+
+#endif // STOREKITCONTROLLER_H