[in_app_purchase_storekit] Add support for macOS (flutter#6517)

* Initial commit of adding MacOS support.

Heavily inspired by flutter#5854

* Updated version and changelog.

* Updated documentation.

* Added missing license comments.

* Fixed podspec lint issue.

* Moved native tests to a shared location.

* Decreased minimum macOS version from 10.15 to 10.11.

This seems wrong to me, since StoreKit is only properly supported on MacOS 10.15+.
However, now all existing tests should pass.

* Added RunnerTests target to macos example.

* Unified macOS capitalization.

* Deleted generated macOS icon assets.

* Removed empty function overrides.

* Enabled tests for macOS.

* Added OCMock to RunnerTests target.

* Adapted tests for macOS.

* Reverted relative path dependencies.

* Updated to a proper version bump.

* Make macOS no-op tests run only on iOS.

* Replace directory symlinks with file symlinks.

* Marked iOS-only API as iOS-only.

Previously they were no-ops on macOS.

* Re-worded doc-comment.

* Formatted code.

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.4
+
+* Adds macOS as a supported platform.
+
 ## 0.3.3
 
 * Supports adding discount information to AppStorePurchaseParam. 

packages/in_app_purchase/in_app_purchase_storekit/README.md

@@ -1,6 +1,6 @@
 # in\_app\_purchase\_storekit
 
-The iOS implementation of [`in_app_purchase`][1].
+The iOS and macOS implementation of [`in_app_purchase`][1].
 
 ## Usage
 

packages/in_app_purchase/in_app_purchase_storekit/example/ios/RunnerTests/FIAPPaymentQueueDeleteTests.m

@@ -0,0 +1 @@
+../../shared/RunnerTests/FIAPPaymentQueueDeleteTests.m

packages/in_app_purchase/in_app_purchase_storekit/example/ios/RunnerTests/FIATransactionCacheTests.m

@@ -0,0 +1 @@
+../../shared/RunnerTests/FIATransactionCacheTests.m