Bridge clients should get directory documents via IPv6
In directory_get_from_dirserver, a bridge client determines the bridge's address using node_get_addr, which only returns IPv4 addresses.
After #17840 (moved) is merged, the bridge client should:
- respect ClientUseIPv4/6
- choose a random directory guard that has an available address family (this should just work in choose_random_dirguard after #17840 (moved), but we need to check this)
- choose an address from that dirguard from an available address family
- respect ClientPreferIPv6OR/DirPort
- set ClientPreferIPv6OR/DirPort to 1 by default?
- this will be tricky because these options already have a default value, and we don't want to write it out