Migrate away from `grpc-web`

[Twey]

gRPC-Web is no longer going to be maintained and will not receive support for client→server streaming, a feature we've been looking forward to to safely overcome the gRPC message size limit.

Let's investigate an alternative that supports bidirectional streaming.

The grpc-web recommended migration path is to grpc-gateway, but:

• officially grpc-gateway doesn't support bidirectional streaming, even though it seems to de facto
• using JSON instead of Protobuf will add some network transfer overhead
• we would have to build a REST client in linera-rpc — we would no longer be able to use the gRPC client on the Web just by changing the transport