Update GTMAppAuth to use AppAuth subspecs

[jwells89]

Hello,

Recently, AppAuth was updated to feature two CocoaPods subspecs – one intended for use in normal iOS apps and one for use in app extensions (which lack access to SafariServices, among other things). Right now, GTMAppAuth does not take advantage of this, making it difficult to use in app extensions (unless I've missed something).

Are there plans to address this issue? With a brief glance, the involved work appears to be fairly minimal, mostly consisting of referencing AppAuthCore.h instead of AppAuth.h when the AppAuth Core subspec is in use.