Port grpc-web interop tests from old repo

[johanbrandhorst]

I'm very pleased to see interop tests are now part of the official repo, and I can deprecate my old interop test repo. However, there are still a few clients and proxies represented in this repo which haven't yet been ported across, for example:

• The Improbable grpc-web client.
• The Improbable Go grpc-web proxy (standalone and in-process).
• The akka-grpc Scala proxy (credit @timw),
• The grpcwsgi Python proxy (credit @public).

Fortunately, all of these should be quite easy to port, as they are already packaged in independent Dockerfiles, with a setup much like the one here.

It would also be nice if we could port the existing compatibility status matrix, since I think it gives a nice overview, and would be nice to carry forward.

[timw]

Minor correction, akka-grpc implements grpc-web directly in its gRPC stack, so it's not a proxy.
i.e. it will require a reimplementation of the echo service used by the tests in akka-grpc.

In the old interop repo, I implemented this entirely in the Docker build so it's self contained - I'm happy to port this if there's consensus to include interop with other server implementations.