Add IPv6 traffic graphs to Relay Detail metrics site

Add IPv6 traffic graphs to Relay Detail metrics site, at least for Relays that report them, that is: ipv6-write-history & ipv6-read-history (non-zero) and have IPv6 OrPort or Flags; ReachableIPv6 or IPv6 Exit.

If you as Relay operator can see, how many usages actually you get on IPv6, it could hopefully increase the migration to IPv6.

This will help us answer questions like:

"How many bytes of Traffic from my Relay travel over IPv6 in percentage compared to IPv4?"

Related: #23761 (closed)