Support new "public" field in topo file

[tzaeschke]

See scionproto/scion@2798312

E.g.

{
  "attributes": [
  "attributes": [
    "core"
    "core"
  ],
  ],
  "isd_as": "1-ff00:0:1",
  "isd_as": "1-ff00:0:1",
  "mtu": 1400,
  "mtu": 1400,
  "border_routers": {
  "border_routers": {
    "br1a": {
    "br1a": {
      "internal_addr": "10.123.10.1:30042",
      "internal_addr": "10.123.10.1:30042",
      "interfaces": {
      "interfaces": {
        "2": {
        "2": {
          "underlay": {
          "underlay": {
            "public": "10.123.2.1:50000",
            "local": "10.123.2.1:50000",
            "remote": "10.123.2.2:50000"
            "remote": "10.123.2.2:50000"
          },
          },
          "isd_as": "1-ff00:0:2",
          "isd_as": "1-ff00:0:2",
          "link_to": "child",
          "link_to": "child",
          "mtu": 1280
          "mtu": 1280
        },
        },
        "3": {
        "3": {
          "underlay": {
          "underlay": {
            "public": "10.123.3.1:50000",
            "local": "10.123.3.1:50000",
            "remote": "10.123.3.3:50000"
            "remote": "10.123.3.3:50000"
          },
          },
          "isd_as": "1-ff00:0:3",
          "isd_as": "1-ff00:0:3",
          "link_to": "child",
          "link_to": "child",
          "mtu": 1280
          "mtu": 1280
        }
        }
      }
      }
    },
    },
    "br1b": {
    "br1b": {
      "internal_addr": "10.123.10.2:30042",
      "internal_addr": "10.123.10.2:30042",
      "interfaces": {
      "interfaces": {
        "4": {
        "4": {
          "underlay": {
          "underlay": {
            "public": "10.123.4.1:50000",
            "local": "10.123.4.1:50000",
            "remote": "10.123.4.4:50000"
            "remote": "10.123.4.4:50000"
          },
          },
          "isd_as": "2-ff00:0:4",
          "isd_as": "2-ff00:0:4",
          "link_to": "core",
          "link_to": "core",
          "mtu": 1280
          "mtu": 1280
        }
        }
      }
      }
    }
    }
  }
  }
}
}