Accept custom providers in the configuration

[elcuro]

Allow to enable/disable custom providers in the config sitemap.providers. Only core providers can be enabled/disabled now.

InvalidConfigurationException

Unrecognized option "cms_sections" under "sylius_sitemap.providers". Available options are "products", "static", "taxons".

It will be useful for plugins which works with the sitemap plugin. E.g. https://github.com/BitBagCommerce/SyliusCmsPlugin/blob/master/doc/sitemap.md

[stefandoorn]

Sounds useful @elcuro, but I doubt if this shouldn't be the responsibility of the other plugin to enable/disable this, or register/unregister it. I'm trying to think of a way to handle this best while keeping configuration sane. Do you have a suggestion and are you able to provide a PR?