Add the Admin extension that enables editing HTML attributes

Contains an interface that encapsulates the attributes related methods for the menuNode.
Relies on burgov/key-value-form-bundle for the admin.
It is nice to check if the Bergov bundle is installed to avoid userland confusion but in terms of enabling this extension the user HAS to change the sonata config (unless we prepend to the config the configuration for applying this to admins). I'm wondering if we should just remove the edit_attributes config altogether since it doesnt actually enable the extension and it shouldn't be necessary to remove it from the container if its not being used (publish workflow extension is not removed for example if you dont use it)