Support dick-typed attachments in `add_attachment` methods

[adubovik]

This facilitates the integration of the DIAL SDK with the DIAL Client without requiring an actual dependency between the two.

See the method that needs to be relaxed:

ai-dial-sdk/aidial_sdk/utils/_attachment.py

Lines 21 to 27 in e673682

	def create_attachment(*args, **kwargs) -> Attachment:
	if args and isinstance(args[0], Attachment):
	return cast(Attachment, args[0])
	elif isinstance(kwargs.get("attachment"), Attachment):
	return cast(Attachment, kwargs.get("attachment"))
	else:
	return _attachment_from_fields(*args, **kwargs)

Don't forget to add unit tests that call add_attachment method on the user-defined MyAttachment class with and without extra fields such as index.