Move to Sylius attributes for strategy configuration

[shochdoerfer]

Currently, the URL resolving strategies are hardcoded in various parts of the plugin. Switch to an attribute-style of configuration to allow others to easily extend and add custom strategies.

[shochdoerfer]

The best(?) approach might be to remove the Strategy creation logic from the Whitelist entity and replace it with a custom Doctrine type:

• https://www.doctrine-project.org/projects/doctrine-dbal/en/4.0/reference/types.html
• https://dev.to/rubenrubiob/auto-registering-custom-doctrine-types-in-symfony-3958

This approach seems ideal as it would not cause a BC break as far as I can tell right now.