fw/output: add metadata attribute to Result

Metadata is a key-value mapping for arbitrary data, similar to
classifiers. Unlike classifiers, metadata does not directly relate to
the results of the execution, but to the execution itself, and typically
would not be processed by Output Processors in the same way as
classifiers. Metadata can also be a lot more free-form in it's value
structure; while classifier values are simple scalars, metadata values
can be arbitrary POD structures.

wa/framework/execution.py

@@ -222,6 +222,12 @@ class ExecutionContext(object):
     def add_event(self, message):
         self.output.add_event(message)
 
+    def add_metadata(self, key, *args, **kwargs):
+        self.output.add_metadata(key, *args, **kwargs)
+
+    def update_metadata(self, key, *args):
+        self.output.update_metadata(key, *args)
+
     def take_screenshot(self, filename):
         filepath = self._get_unique_filepath(filename)
         self.tm.target.capture_screen(filepath)