Implement outputting of IPv6 addresses in the ippool debug list of tree
type pools (ippool -l -d -t tree). Currently IPv6 in ippool tree type
pool handling is partially implemented (meaning it doesn't work).
This is the first of a series of commits to remediate ippool.

This will be MFCed with a yet to be committed series of fixes to ippool
after it has been fully remediated.

PR: 218433