feat: add libtransmission::api_compat::convert() (#7917)

Add a module to convert between tr4 and tr5 RPC / settings / config file payloads.

This will be used as a compatibility layer between Transmission 5's naming scheme
and Transmission <= 4.

Co-authored-by: Yat Ho <lagoho7@gmail.com>
Co-authored-by: Dzmitry Neviadomski <nevack.d@gmail.com>

libtransmission/rpcimpl.h

@@ -5,6 +5,7 @@
 
 #pragma once
 
+#include <cstdint> // int16_t
 #include <functional>
 
 struct tr_session;
@@ -34,7 +35,9 @@ enum Code : int16_t
     HTTP_ERROR,
     CORRUPT_TORRENT
 };
-}
+
+[[nodiscard]] std::string_view to_string(Code code);
+} // namespace Error
 } // namespace JsonRpc
 
 using tr_rpc_response_func = std::function<void(tr_session* session, tr_variant&& response)>;