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        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/536