networkd-json: add human-readable address strings for IP addresses

networkd-json: add human-readable address strings for IP addresses

Add JSON_BUILD_PAIR_IN_ADDR_WITH_STRING, JSON_BUILD_PAIR_IN6_ADDR_WITH_STRING,
and JSON_BUILD_PAIR_IN4_ADDR_WITH_STRING macros (plus _NON_NULL variants) that
emit both the binary address array and a human-readable string representation.

For example, an address field now emits both:
  "Address": [192, 168, 1, 1]
  "AddressString": "192.168.1.1"

This improves usability of the JSON/Varlink output for debugging and tooling
that consumes networkd state.

Updated networkd-json.c to use these new macros for addresses, neighbors,
nexthops, routes, routing policy rules, DNS servers, NTP servers, SIP servers,
domains, DHCPv6 prefixes, and link-local addresses.

Also updated the Varlink interface definitions to include the new string fields.