Add support for GTMAppAuth 5 on macOS #762

Job	Run time
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec, --use-static-frameworks)	3m 41s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec)	3m 40s
spm-build-test (macos-latest, macosx)	55s
spm-build-test (macos-latest, iphonesimulator)	43s
pod-lib-lint (macos-13, GoogleSignIn.podspec)	6m 31s
pod-lib-lint (macos-13, GoogleSignIn.podspec, --use-static-frameworks)	3m 46s
	19m 16s

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