[OptionsResolver] Allow giving a callback as an allowedValue to OptionsResolver

I recently had to use an option which was an array and could contain some one or multiple values from a list. As it could contain all possible combinations, it was not possible to validate it with a list of allowed values.

| 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/symfony-docs/pull/3437