Update MessageEntry to support sending multiple entries at once

[woswos]

This was the draft version of MessageEntry before the unified messages class. First understand and remember what we were targeting. Later, find a nice way to integrate it into the class.

class MessageEntry {
   public:
      MessageType type = ENTRY;
      
          string_t serialize(std::vector<std::pair<uint32_t, string_t>> entries) {
          
                DynamicJsonDocument payload(MESSAGE_REQUEST_APPEND_ENTRY_PAYLOAD_SIZE);
                
                string_t serialized_payload;
                payload["type"] = type;
                payload["entries"] = entries;
                
                serializeJson(payload, serialized_payload);
          
          return serialized_payload;
    }
  };