rpc: add "warnings" field to RPCs {create,load,unload,restore}wallet

This new "warnings" field is a JSON array of strings intended to replace the
"warning" string field in these four RPCs, to better handle returning multiple
warning messages and for consistency with other wallet RPCs.

src/rpc/util.h

@@ -388,5 +388,6 @@ private:
  * @param[out] obj      UniValue object to push the warnings array object to.
  */
 void PushWarnings(const UniValue& warnings, UniValue& obj);
+void PushWarnings(const std::vector<bilingual_str>& warnings, UniValue& obj);
 
 #endif // BITCOIN_RPC_UTIL_H