[SignalR] Identify a suitable MessagePack library for the Java client to use

[analogrelay]

Epic #21022

Before we can actually implement the MessagePack Hub Protocol in the Java Client, we need to identify a suitable MessagePack library. Some of our requirements:

• Open-Source with permissive license.
• Must be relatively well-adopted.
• Must support the necessary data types described in Hub Protocol (we don't use anything funky so this should be straightforward)
• Ideally, it should support a "reader/writer" style API (like Utf8JsonWriter and Utf8JsonReader), a serialization API (like JsonSerializer) and support interop (i.e. be able to inject a serialized object into the output while using the writer API, and vice-versa for reading). This allows us higher performance without compromising user flexibility, and allows the user to configure the serializer without messing with the SignalR framing.

[wtgodbe]

The official Java implementation looks promising: https://github.com/msgpack/msgpack-java ("official" meaning "linked to from the messagepack website": https://msgpack.org/index.html)

• License is Apache 2.0
• Core class provides an API for deserializing objects from binary, MessageUnpacker
• Core class provides an API for serializing objects from binary, MessageBufferPacker and MessagePacker
• MessagePacker writes to a MessageBufferOutput, implementing classes of which can write to an Array, Channel or OutputStream (pre-existing types from java.io)
• MessageUnpackager reads from MessageBufferInput, which has similar properties to MessageBufferOutput

Interop therefore seems baked in. It doesn't appear to have a static serializer like JsonSerializer, though. Still poking around.

[BrennanConroy]

Should be good to go, found a library