message: Add missing function overloads
diff: @@ -53,6 +53,9 @@ public:
  /// An iterator over ::config_option unique pointers.
  using iterator = option_vector::iterator;
  + /// An iterator over ::config_option unique pointers.
  + using const_iterator = option_vector::const_iterator;
  +
  /// Maps string keys to arbitrary (config) values.
  using dictionary = caf::dictionary<config_value>;
  @@ -89,11 +92,21 @@ public:
  return opts_.begin();
  }
  + /// Returns an iterator to the first ::config_option object.
  + const_iterator begin() const noexcept {
  + return opts_.begin();
  + }
  +
  /// Returns the past-the-end iterator.
  iterator end() noexcept {
  return opts_.end();
  }
  + /// Returns the past-the-end iterator.
  + const_iterator end() const noexcept {
  + return opts_.end();
  + }
  +
  /// Adds a config option to the set.
  template <class T>
  config_option_set& add(string_view category, string_view name,
  