Consider adding TypeReference class to MessagePack

[wtgodbe]

Followup to #21644 and #23532

We require users to provide a java Type when calling HubConnection.on() and HubConnection.invoke(). We should provide a utility for users to get a Type from a Class, like TypeReference (as described here http://gafter.blogspot.com/2006/12/super-type-tokens.html), but we should make sure it's kosher to do so first.

[wtgodbe]

Resolved by #25286