Handle set elements that are structured prefixes

[caseydavenport]

For sets that use the "interval" flag, it is possible to include prefixes as elements. In JSON representation, these are returned like so:

{
  "nftables": [
    {
      "metainfo": {
        "version": "1.0.2",
        "release_name": "Lester Gooch",
        "json_schema_version": 1
      }
    },
    {
      "set": {
        "family": "ip",
        "name": "cali40all-ipam-pools",
        "table": "calico",
        "type": "ipv4_addr",
        "handle": 224,
        "flags": [
          "interval"
        ],
        "elem": [
          {
            "prefix": {
              "addr": "192.168.0.0",
              "len": 16
            }
          }
        ]
      }
    }
  ]
}

This change updates ListElements to return these as strings in CIDR notation to match how they are represented when creating the element.

[aojea]

I see this is documented https://manpages.debian.org/testing/libnftables1/libnftables-json.5.en.html

PREFIX
{ "prefix": {


        "addr": EXPRESSION,


        "len": NUMBER
}}
Construct an IPv4 or IPv6 prefix consisting of address part in addr and prefix length in len.

[aojea]

/lgtm

[danwinship]

/approve

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