Add External API endpoints for BGP unnumbered interface manager status

[taspelund]

This is the omicron side of maghemite#800.

In mgd, we are refactoring the internals of BGP unnumbered which affects the mg-api by no longer requiring the BGP ASN for querying the state... However, omicron never had plumbing to query BGP unnumbered interface state to begin with, which is what this PR remedies.

The new API endpoints are purely status, with no config whatsoever.

The maghemite hash currently points to the feature branch, but will be updated to main once this PR's counterpart lands.

[taspelund]

I have taken this through a4x2 manually and have validated that e2e connectivity works, as do the new endpoints:

trey@korgano 03:21:19 AM | ~/git/omicron ➦ d372d6342 ○
‣ curl -sS "${AUTH[@]}" "http://198.51.100.24/v1/system/networking/bgp/unnumbered/manager" | jq
[
  {
    "switch_slot": "switch0",
    "state": {
      "monitor_running": true,
      "pending_interfaces": [],
      "active_interfaces": [
        "qsfp0",
        "qsfp1"
      ]
    }
  },
  {
    "switch_slot": "switch1",
    "state": {
      "monitor_running": true,
      "pending_interfaces": [],
      "active_interfaces": [
        "qsfp1",
        "qsfp0"
      ]
    }
  }
]

trey@korgano 03:21:22 AM | ~/git/omicron ➦ d372d6342 ○
‣ curl -sS "${AUTH[@]}" "http://198.51.100.24/v1/system/networking/bgp/unnumbered/interfaces" | jq
[
  {
    "switch_slot": "switch0",
    "interface": {
      "interface": "qsfp0",
      "local_address": "fe80::aa40:25ff:fe25:9735",
      "router_lifetime": 1800,
      "discovered_peer": {
        "address": "fe80::aa40:25ff:fe00:9",
        "discovered_at": "2026-07-01T00:28:26Z",
        "last_advertisement": "2026-07-01T03:23:15Z",
        "router_lifetime": 30,
        "reachable_time": 0,
        "retrans_timer": 0,
        "expired": false,
        "time_until_expiry": "9s 61ms"
      },
      "ndp_state": {
        "tx": true,
        "rx": true
      }
    }
  },
  {
    "switch_slot": "switch0",
    "interface": {
      "interface": "qsfp1",
      "local_address": "fe80::aa40:25ff:fe25:9739",
      "router_lifetime": 1800,
      "discovered_peer": {
        "address": "fe80::aa40:25ff:fe00:11",
        "discovered_at": "2026-07-01T00:28:26Z",
        "last_advertisement": "2026-07-01T03:23:15Z",
        "router_lifetime": 30,
        "reachable_time": 0,
        "retrans_timer": 0,
        "expired": false,
        "time_until_expiry": "9s 398ms"
      },
      "ndp_state": {
        "tx": true,
        "rx": true
      }
    }
  },
  {
    "switch_slot": "switch1",
    "interface": {
      "interface": "qsfp1",
      "local_address": "fe80::aa40:25ff:fe4f:cc12",
      "router_lifetime": 1800,
      "discovered_peer": {
        "address": "fe80::aa40:25ff:fe00:12",
        "discovered_at": "2026-07-01T00:24:15Z",
        "last_advertisement": "2026-07-01T03:23:16Z",
        "router_lifetime": 30,
        "reachable_time": 0,
        "retrans_timer": 0,
        "expired": false,
        "time_until_expiry": "9s 550ms"
      },
      "ndp_state": {
        "tx": true,
        "rx": true
      }
    }
  },
  {
    "switch_slot": "switch1",
    "interface": {
      "interface": "qsfp0",
      "local_address": "fe80::aa40:25ff:fe4f:cc0e",
      "router_lifetime": 1800,
      "discovered_peer": {
        "address": "fe80::aa40:25ff:fe00:10",
        "discovered_at": "2026-07-01T00:24:15Z",
        "last_advertisement": "2026-07-01T03:23:16Z",
        "router_lifetime": 30,
        "reachable_time": 0,
        "retrans_timer": 0,
        "expired": false,
        "time_until_expiry": "9s 549ms"
      },
      "ndp_state": {
        "tx": true,
        "rx": true
      }
    }
  }
]

trey@korgano 03:24:15 AM | ~/git/omicron ➦ d372d6342 ○
‣ curl -sS "${AUTH[@]}" "http://198.51.100.24/v1/system/networking/bgp/unnumbered/interfaces/switch0/qsfp0" | jq
{
  "switch_slot": "switch0",
  "interface": {
    "interface": "qsfp0",
    "local_address": "fe80::aa40:25ff:fe25:9735",
    "router_lifetime": 1800,
    "discovered_peer": {
      "address": "fe80::aa40:25ff:fe00:9",
      "discovered_at": "2026-07-01T00:28:26Z",
      "last_advertisement": "2026-07-01T03:27:15Z",
      "router_lifetime": 30,
      "reachable_time": 0,
      "retrans_timer": 0,
      "expired": false,
      "time_until_expiry": "9s 260ms"
    },
    "ndp_state": {
      "tx": true,
      "rx": true
    }
  }
}