fable: Add get_factory_keys() to Factory schema

This allows users to write better error messages.

Author: cassava

fable/include/fable/schema/factory.hpp

@@ -223,6 +223,17 @@ class FactoryBase : public Base<CRTP> {
     return available_.at(key);
   }
 
+  /**
+   * Return the set of added factory keys.
+   */
+  [[nodiscard]] std::vector<std::string> get_factory_keys() const {
+    std::vector<std::string> keys;
+    for (const auto& [key, value] : available_) {
+      keys.emplace_back(key);
+    }
+    return keys;
+  }
+
   /**
    * Return whether a factory with the given key is available.
    */