FR: macOS Firestore support

[PierBover]

As per Salman Qadri's suggestion I'm opening this FR.

[GetyUp]

With Firestore going GA yesterday, macOS support would be fantastic!

[wilhuff]

Until Firebase more broadly supports macOS, macOS support is a community endeavor. That said, #2241 made the required changes and added support in the podspec. This went out with Firestore 1.0.0 as a part of the 5.16 release of Firebase.

This FR remains open because we still haven't gotten this fully integrated with our CI environment, but it should work.

[wilhuff]

OK tests working (#2641) and running on Travis (#2645) is good enough to call this done.