firebase-ios-sdk

Open source the Swizzler (#1564)

* Import the Swizzler sources into github

* Add subspecs to GoogleUtilities podspec

* Add post install step to test Podfile to add the preprocessor flags required for testing.

* Add the swizzler tests (except the flaky GULAppDelegateSwizzlerTest) to the Xcode project

* Add the AppDelegateSwizzler internal headers to the ios test header search path

* Add license header to GULLoggerCodes.h

* Fix/Suppress warnings in the framework.

* Disable certain classes for tvOS and macOS as well fix some additional pod lib lint warnings.

* Modify imports as well as add the required preprocessor flags to the test targets.

* Run pod deintegrate.