utils/program-options: selection_value: make get_candidate_names() pu…

…blic

To allow application code query available candidates, before making a
choice.

Closes #2550

include/seastar/util/program-options.hh

@@ -534,7 +534,7 @@ private:
     size_t _selected_candidate = no_selected_candidate;
     bool _defaulted = true;
 
-private:
+public:
     std::vector<std::string> get_candidate_names() const {
         std::vector<std::string> candidate_names;
         candidate_names.reserve(_candidates.size());
@@ -543,6 +543,7 @@ private:
         }
         return candidate_names;
     }
+private:
     virtual void do_describe(options_descriptor& descriptor) const override {
         descriptor.visit_selection_value(get_candidate_names(), _selected_candidate == no_selected_candidate ? nullptr : &_selected_candidate);
         for (auto& c : _candidates) {