firebase-ios-sdk

Upgrade gRPC-C++ to 1.28.0 (#4312)

This unblocks nanopb changes proposed in #4264.

This upgrades everything in the gRPC constellation of dependencies to match gRPC at 1.28:

  * gRPC to 1.28.0 (note that gRPC 1.28.1, while released, is a fix for just the python binding and there's no gRPC-C++ CocoaPod at that version)
  * Abseil to 20200225 (as of gRPC 1.27 this is now a gRPC dependency too; there's a later patch but gRPC doesn't use it yet)
  * Protobuf C++ to 3.11.4 (only used for testing in Firestore)
  * C-Ares to 1.15.0
  * BoringSSL to match (no change between gRPC 1.27.0 and 1.28.0)

Also, this disables CFStream-based transport on Apple platforms. This works around b/133182964, wherein CFStream will occasionally fail to raise a has-bytes-available events, causing Firestore to appear to hang.