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 would prefer the explicit configuration option, and default to false. that way we document that you have to enable it, and if somebody messes up their composer.json we can throw an explicit exception that burgov is missing. if we just implicitly look whether the bundle is loaded, the error will only occur when trying to instantiate an admin that has this extension configured, and the error will say that some service is not found, without any explanation what went wrong.