message: core: move 2 methods to public interface
  is_connected and get_uuid are convenient to have for a system.
diff: @@ -62,6 +62,20 @@ public:
  */
  bool has_gimbal() const;
  + /**
  + * @brief Checks if the system is connected.
  + *
  + * A system is connected when heartbeats are arriving (discovered and not timed out).
  + * @return `true` if the system is connected.
  + */
  + bool is_connected() const;
  +
  + /**
  + * @brief Get the UUID of the system.
  + *
  + * @return UUID of system.
  + */
  + uint64_t get_uuid() const;
  // Non-copyable
  /**
  @@ -79,8 +93,6 @@ private:
  void add_new_component(uint8_t component_id);
  void process_mavlink_message(const mavlink_message_t &message);
  void set_system_id(uint8_t system_id);
  - bool is_connected() const;
  - uint64_t get_uuid() const;
  uint8_t get_system_id() const;
  std::shared_ptr<MAVLinkSystem> mavlink_system() { return _mavlink_system; };
  