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 agree. it's gonna be hard for user to find out they get the error because the Burgov bundle is missing.
@dbu you mentioned one situation that if they have don't have Sonata admin and they still wants to use this service. Is it really possible? I think I have to throw the same type of exception if the Sonata admin was not available but the edit_attributes was set to true.