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

And maybe:

• 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