[RFC] make checkbox_url_label work with any symfony route

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n.a.
| License       | MIT
| Doc PR        | see https://github.com/symfony-cmf/symfony-cmf-docs/issues/189

this PR makes the form type more generic but also more complex to use, see Tests/Functional/Form/CheckboxUrlLabelFormTypeTest.php