firebase-ios-sdk

Delete Firestore public C++ API (#2050)

* Add firebase_cpp_sdk 5.4.0

... to the Firestore CMake build.

* Move C++ public API headers to cpp/include

* Add Firebase C++ API tests.

* Add support for building on Linux

* Add clang-format configuration for Firestore/cpp

* Add windows build support

* Review feedback

* Revert build changes to support Firebase C++ integration.

It turns out the public Firebase C++ SDK doesn't include enough to
actually build another component of that SDK externally so these changes
don't really help.

Eventually, once Firebase C++ is open source we'll integrate with that
to actually test Firestore's public C++ API.

* Delete Firestore/cpp and public C++ artifacts

It's not yet feasible to build these components externally so there's no
use in keeping them here.