[http-client-java] use BinaryData for typespec unknown

[weidongxu-microsoft]

relates to https://github.com/Azure/autorest.java/issues/2055

After Java migration to azure-json, we do not want to depends on Jackson to do serialization/de-serialization. Therefore we'd depends more on the BinaryData class and other helper method from azure-json lib to do the serialization/de-serialization.

And in such context, we should use BinaryData for unknown for DPG/TypeSpec.