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.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO
I think it works the same way as it works for KnpMenu. So the attibutes will be passed to the MenuFactory, HTML or non-HTML. It isn't documented in Knp\Menu\itemInterface that it's only HTML.