Wraps std::shared_ptr<EncodingParamsImpl> on the C++ side.
Returned by get_encoding_params(cc) and by
Plaintext::GetEncodingParams() once the corresponding
Plaintext accessor lands. This class ships as scaffolding
only: the S7 class definition is in place so that the getter
wiring can treat it as already defined, but there is no
constructor path from R.