Allow the user to use whatever gRPC implementation they want.

[YoEight]

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Changed: Allow the user to use whatever gRPC implementation they want.

Fixes #249

This patch allows the user to use whatever gRPC implementation they want which is valuable if you want to use the client in Android for example. As a consequence, users will have to import grpc-netty, grpc-netty-shaded or grpc-okhttp depending on their situation.

[YoEight]

I would put that PR on hold because I'm unable to disable certificate verification using Grpc.newChannelBuilderForAddress and TlsChannelCredentials only. Setting a custom insecure trust manager and a hostname verifier do absolutely nothing. Will investigate what the folks working on grpc-netty-shaded did to make that possible.