Nonrequired embedded resources

Based off my other PR, it would be nice to allow for embedded resources to sometimes not be required.
As long as it makes sense. We don't know how ppl are going to use the library.
Let's think:
Now it is always True (Attr has a default required True). So it will raise if there's no attr to embed.
I can imagine when you want to drop it. Especially in the case of scalar embedded.
There's also default not passed along. But I'm not sure what can I imagine for the default. None for scalar or empty list for lists?
I guess we could bypass these parameters to the superclass.