The third party setting config for rocketship_seo needs to have a proper schema

This is ready for review.

The seo_fields property is not a mapping, it is a list (potentially empty) of fields that need to be in the SEO wrapper.

https://git.drupalcode.org/project/rocketship_core/-/merge_requests/148/... needs to be changed

@l_vandamme I hope keeping the seo_fields as a sequence type will be fine. I have updated the MR; please review.