Missing option to control mask size ipv6 address
According to https://www.ripe.net/publications/docs/ripe-690#4-1-1---64-prefix-from-a-dedicated-pool-of-ipv6-prefixes recommended config is to assign /64 address to wan interface and then /56 pd behind it, but in KEA dhcp6 the option to control the size of mask is missing subnet/pool.