Add support for web apps

[nickmeinhold]

As described in #47 the firedart package doesn't support web apps because browsers only speak grpc-web, not gRPC. grpc-dart does have support for grpc-web and offers grpc-dart/lib/grpc_or_grpcweb.dart.

I don't know if the googleapis endpoints speak grpc-web and I can't find anything about it either way but if not the user can create a proxy with envoy as described in Basics tutorial | Web | gRPC