[FR] For macOS, don't unnecessarily stub out iOS+ SDK

Hey wonderful Firebase folks,

The iOS+ SDK these days has [pretty broad support for macOS](https://firebase.google.com/docs/ios/learn-more#firebase_library_support_by_platform). But on quick inspection, it looks like a lot of that functionality, e.g., Analytics, is unnecessarily stubbed out in this C++ SDK.

Could you fix that? It seems like it wouldn't be that hard to change the build to pipe it through, same as tvOS, since the underlying pieces are there.

Thanks for your consideration!
Chris
(ex-Googler)

P.S. If it's already supported--and I've missed it, then please correct me, and my apologies in advance.
P.P.S. #234 is related in that both involve desktop analytics support, but this issue differs in that it should be quite easy to add macOS support more broadly, since it's already supported by an underlying platform SDK.

No files or tests are named. Start by inspecting the platform conditionals and build configuration for the iOS+ SDK, comparing the macOS path with tvOS; done means macOS builds route supported Firebase functionality through instead of using unnecessary stubs.

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