Add addl support for rules reader/compiler subclasses

To support subclasses that may extend the falco rules format, add
additional error/warning/item types for an extension item.

When subclasses report errors and warnings, they can use these
codes/item types in context objects and still provide an exact
line/column context.

Also make some previously static functions in rules reader protected
methods so they can be used in sub-classes.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/rule_loader_reader.h

@@ -57,6 +57,19 @@ public:
 			+ std::to_string(minor) + "." 
 			+ std::to_string(FALCO_ENGINE_VERSION_PATCH));
 	}
+
+	template <typename T>
+	static void decode_val(const YAML::Node& item, const char *key, T& out, const rule_loader::context& ctx);
+
+	template <typename T>
+	static void decode_optional_val(const YAML::Node& item, const char *key, T& out, const rule_loader::context& ctx);
+
+protected:
+
+	virtual void read_item(rule_loader::configuration& cfg,
+			       rule_loader::collector& collector,
+			       const YAML::Node& item,
+			       const rule_loader::context& parent);
 };
 
 }; // namespace rule_loader