rpc-client-core: Always re-encode requests and responses, and preserve unrecognized struct fields.
We want to preserve unrecognized struct fields to keep arti-rpc-client-core
compatible with future RPC protocol updates.
We want to ensure that everything is re-encoded to limit any opportunities for security bugs (or other bugs) resulting from different interpretations of dubious JSON from arti, arti-rpc-client-core, and the applications. (See #1491 (closed) for discussion on this.)
Closes #1491 (closed).