Add CLI support for Discovery of Network-designated Resolvers (DNR)
under DHCP option trees and render it to Kea option-data.

• Add new DNR nodes for DHCPv4 and DHCPv6 option config:
  • priority
  • authentication-domain-name
  • address (v4/v6)
  • service-parameter (alpn/port/dohpath/raw)
• Map DNR config in python/vyos/kea.py:
  • DHCPv4: emit a single v4-dnr option with instances joined by "|"
  • DHCPv6: emit one v6-dnr option per instance
• Add smoke tests for DHCPv4 and DHCPv6 DNR rendering.

Note: SLAAC/RA DNR is not part of this commit; current router-advert
backend (radvd) does not expose RFC9463 DNR option support.