Add TMaybeList as a new type for parameters

This will be used as the type for the reason trail parameter.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

lib/ht.py

@@ -356,6 +356,9 @@ TMaybeBool = TMaybe(TBool)
 #: Maybe a dictionary (dict or None)
 TMaybeDict = TMaybe(TDict)
 
+#: Maybe a list (list or None)
+TMaybeList = TMaybe(TList)
+
 #: a non-negative integer (value >= 0)
 TNonNegativeInt = \
   TAnd(TInt, WithDesc("EqualOrGreaterThanZero")(lambda v: v >= 0))