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/falco_load_result.h

@@ -34,7 +34,8 @@ public:
 		LOAD_ERR_YAML_VALIDATE,
 		LOAD_ERR_COMPILE_CONDITION,
 		LOAD_ERR_COMPILE_OUTPUT,
-		LOAD_ERR_VALIDATE
+		LOAD_ERR_VALIDATE,
+		LOAD_ERR_EXTENSION
 	};
 
 	// The error code as a string
@@ -55,7 +56,8 @@ public:
 		LOAD_UNUSED_MACRO,
 		LOAD_UNUSED_LIST,
 		LOAD_UNKNOWN_ITEM,
-		LOAD_DEPRECATED_ITEM
+		LOAD_DEPRECATED_ITEM,
+		LOAD_WARNING_EXTENSION
 	};
 
 	virtual ~load_result() = default;