Create a dataclass that supports JSON serialization.

This function defers to the ordinary dataclass decorator but appends the _json_dict_ protocol method which automatically determines the appropriate fields from the dataclass.

namespace An optional prefix to the value associated with the key "cirq_type". The namespace name will be joined with the class name via a dot (.) init, repr, eq, order, unsafe_hash, frozen: Forwarded to the dataclass constructor.