MessagePackFormatter fails to deserialize Union types

[AArnott]

Given a type hierarchy that utilizes MessagePack's UnionAttribute:

[MessagePackObject]
[Union(0, typeof(UnionDerivedClass))]
public abstract class UnionBaseType
{
}

[MessagePackObject]
public class UnionDerivedClass : UnionBaseType
{
}

And given a server method that declares to return the base type:

public UnionBaseType ReturnUnionType() => new UnionDerivedClass();

MessagePackFormatter fails to deserialize the result:

UnionBaseType result = await this.clientRpc.InvokeWithCancellationAsync<UnionBaseType>(nameof(MessagePackServer.ReturnUnionType), null, this.TimeoutToken);
Assert.IsType<UnionDerivedClass>(result);

This throws on the Invoke line because the union data was not serialized. MessagePackFormatter serializes the return value as UnionDerivedClass directly, so no union data appears necessary. But on the receiving end, only UnionBaseClass is expected, so union data is necessary.

To fix this, the MessagePackFormatter needs to know what the declared type is and tell the MessagePackSerializer to serialize that as a generic type argument. That way the union data (if any) will be included.

[AArnott]

We need to support this for arguments as well as return values.
While return types have a declared type on both sides, arguments do not have a declared type on the originating side -- only the receiving side declares parameter types.
This leaves us with a very interesting problem for how to send arguments involving Union types with the MessagePackFormatter (or even the JsonMessageFormatter if combined with a custom discriminator JsonConverter, perhaps).