DP: try alternative QN by other providers

[traumschule]

A solution to prevent #3927 could be for the DP to get a list of QN or optimistically try a default URL for registered providers that are nearby (respond earliest).

• Where to get a list of official QN that work properly (#3677)?
• DP endpoints are registered on-chain and can be used to append /graphql to the domain as first guess
• DP already track response times for other providers
• on startup optimistically test for a local QN and if it fails for some reason try other ones