Relax type enforcement with custom argparse actions

This code warns the user and ignores configuration defaults when the typ
isn't in the expected format. For custom types using a callable 'type'
parameter, this code looks for a string input which is implicitly
converted by argparse to the custom type by calling the given 'type'
conversion function.

For custom actions, the parsed output cannot be known without invoking
the action, which argparse does not do implicitly for given defaults.
When the user specifies a custom action, we should relax the type
enforcement.

This change doesn't bring support for all possible ways to trigger this
issue. For example, a user may register a custom action behind a string
identifier, or a custom action may result in a type which doesn't align
with what 'nargs' says the argument should look like. I'm not aware of
any use of either of those patterns in colcon today, so I think we
should wait to decide how to handle them when we have a real use case to
test against. For now, this change leaves that behavior as-is.

colcon_defaults/argument_parser/defaults.py

@@ -108,6 +108,8 @@ def add_argument(self, *args, **kwargs):
             type_ = int
         elif kwargs.get('action') in ('store_false', 'store_true'):
             type_ = bool
+        elif callable(kwargs.get('action')):
+            type_ = None
         elif 'type' not in kwargs:
             type_ = str
         if kwargs.get('nargs') in ('*', '+'):