Add support for GTMAppAuth 5 on macOS #760

Job	Run time
spm-build-test (macos-latest, macosx)	1m 33s
spm-build-test (macos-latest, iphonesimulator)	1m 47s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec)	15m 55s
pod-lib-lint (macos-13, GoogleSignIn.podspec)	8m 43s
pod-lib-lint (macos-13, GoogleSignInSwiftSupport.podspec, --use-static-frameworks)	14m 10s
pod-lib-lint (macos-13, GoogleSignIn.podspec, --use-static-frameworks)	8m 49s
	50m 57s

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