Dynamic Optimade providers retrieval for white-listing

[blokhin]

Should be done e.g. once per week from https://www.optimade.org/providers/links/

[giovannipizzi]

I think once per day is an even better compromise (the cost is small, and it's acceptable to wait overnight, while a week might be long).
Could even be done more often (every few hours) if the hit on the index databases is small and acceptable.