Add MessagePack support for the Java SignalR Client

[BrennanConroy]

Part of #21022
Continuation of #21107

We'll want to make a new package. com.microsoft.signalr.messagepack maybe?
It'll depend on com.microsoft.signalr.

We'll also need to make the HubProtocol abstraction public, and add an API to the builder to add a custom HubProtocol.

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