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.
i think you need to move this to a separate service config file actually. otherwise if this is enabled but the sonata admin is not (which would make sense when i do my own admin) things will fail. and if both are disabled, they fail as well because the definition will not be present.