Built-in support of grpc-web clients

Supporting grpc-web clients is possible only through a reverse proxy like Envoy which might reduce the performance. Is it possible to add direct support as [this wrapper for grpc-go](https://github.com/improbable-eng/grpc-web/tree/master/go/grpcweb)?

The issue names the grpc-web grpc-go wrapper as the reference implementation but no Java files or tests. Start by reviewing that wrapper and the grpc-java architecture to determine the scope of direct client support. Done means grpc-web clients can connect directly without requiring a reverse proxy such as Envoy.

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