Add info about default values to describe

Change-Id: I9919e0d366b56ac17e7b606aa5aa0ae048b447c3
diff --git a/src/schema.hpp b/src/schema.hpp
index b8baf34..49fc5db 100644
--- a/src/schema.hpp
+++ b/src/schema.hpp
@@ -55,6 +55,7 @@
     virtual bool listHasKey(const schemaPath_& location, const ModuleNodePair& node, const std::string& key) const = 0;
     virtual bool leafIsKey(const std::string& leafPath) const = 0;
     virtual bool isConfig(const std::string& path) const = 0;
+    virtual std::optional<std::string> defaultValue(const std::string& leafPath) const = 0;
     virtual const std::set<std::string> listKeys(const schemaPath_& location, const ModuleNodePair& node) const = 0;
     virtual yang::TypeInfo leafType(const schemaPath_& location, const ModuleNodePair& node) const = 0;
     virtual yang::TypeInfo leafType(const std::string& path) const = 0;