Prop 312: Make relay pick IPv4 or/and IPv6 resolved address in descriptor
At the moment, router_pick_published_address() is used to pick the address we'll put in a relay descriptor. It checks at the last resolved address, then attempts to locally find the address and finally attempts to look at what a directory server suggested us.
With the work in #40022 (closed), we now discover our address with the NETINFO cell for both IPv4 and IPv6. We should use that.
Thus this ticket means that we need a new interface to query "what address should I use in my descriptor" so it is usable per address family and queries the right caches (that is not the directory guessed IP cache anymore).
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