Add support for GTMAppAuth 5 on macOS #759

Job	Run time
spm-build-test (macos-latest, macosx)	1m 21s
spm-build-test (macos-latest, iphonesimulator)	1m 49s
pod-lib-lint (macos-13, GoogleSignIn.podspec, --use-static-frameworks)	16m 22s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec)	10m 12s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec, --use-static-frameworks)	10m 15s
pod-lib-lint (macos-13, GoogleSignIn.podspec)	26m 28s
	1h 6m 27s

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