firebase-ios-sdk

Install protobuf during CMake build (#1460)

* Install protobuf

  * Use the built-in FindProtobuf.cmake
  * Add additional build configuration flags
  * Preserve generator configuration in the protobuf sub-build

    Without this, the build fails on Win64 because the default
    configuration builds a Win32 libprotobuf and the Win64 build rejects
    it.
  * Wire the installed protobuf into the gRPC build.

* Install nanopb