Enhancement: Add support for many_many

[chrispenny]

The mechanism for invalidating keys will all be fine, we just need to update our graph to follow the path for the different many_many definitions.

https://docs.silverstripe.org/en/4/developer_guides/model/relations/#many-many

We already support many_many through if a developer were to simply cares/touches the has_many relationship, but we will need to add support for when that is not the case, and the developer has said they cares/touches the many_many.