Check authority IPv6 addresses using DocTor

After #17840 (moved) is merged, tor clients can bootstrap over IPv6 by setting ClientUseIPv4 0 (or a slightly more complex incantation for dual-stack clients).

Some clients will use the authorities to bootstrap, so can we check authority IPv6 addresses regularly using DocTor?

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information