[google_sign_in] Remove use of OCMock (#10290)

Wraps Google Sign In SDK classes with protocols, so that we can inject fakes that implement the protocols instead of using OCMock to mock the actual classes. This will allow conversion of the tests to Swift, in preparation for migrating the entire plugin to Swift.

For now the protocols are implemented in production with passthrough wrappers, so this adds a fair amount of boilerplate code to the plugin, but almost all of it will be removed when the plugin implementation is converted to Swift since we will be able to use extensions to make the SDK classes conform to the protocol instead of maintaining wrappers.

Unblocks flutter/flutter#119103

## Pre-Review Checklist

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Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_ios/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 6.2.3
+
+* Internal restructuring to improve testability.
+
 ## 6.2.2
 
 * Updates minimum supported version to iOS 13.