feat: add kubectl-us-net plugin for Neutron/OVN troubleshooting

[cardoe]

Adds a kubectl plugin wrapping ovn-nbctl/sbctl/vsctl/appctl and OpenStack
lookups behind router list/show commands, plus operator-guide docs.

[cardoe]

❯ uv run kubectl-us-net router show doug-ovn-patch
================================================================
kubectl-us-net -- target environment
================================================================
  Kubernetes context : my-cluster
  OVN namespace/pods : openstack (nb=ovn-ovsdb-nb-0, sb=ovn-ovsdb-sb-0)
  OpenStack cloud    : (default via OS_CLOUD / clouds.yaml)
    auth URL         : https://keystone.example.com/v3
    project          : baremetal
================================================================


Router doug-ovn-patch (94c6e0ee-3959-49e2-a9f2-e5f3a304bb77)
OVN Logical_Router: neutron-94c6e0ee-3959-49e2-a9f2-e5f3a304bb77
Type: centralized (options:chassis=a2172b59-5558-4018-858d-88947e2d9adf)

Router ports:
  - lrp-6aeee225-e410-4a7a-891d-edc2a3f3831f [gateway]
      Neutron port      : 6aeee225-e410-4a7a-891d-edc2a3f3831f
      OVN networks      : 203.0.113.0/24
      Neutron fixed IPs : 203.0.113.41
      Network VLAN tag  : 1800
      Pinned chassis    : a2172b59-5558-4018-858d-88947e2d9adf (alive, physnets=f20-1-network)
  - lrp-c0a41495-4cb5-4913-a7e7-9a2047f79e7d [internal]
      Neutron port      : c0a41495-4cb5-4913-a7e7-9a2047f79e7d
      OVN networks      : 192.168.20.1/24
      Neutron fixed IPs : 192.168.20.1
      Network VLAN tag  : 1802
      Pinned chassis    : a2172b59-5558-4018-858d-88947e2d9adf (alive, physnets=f20-1-network)

NAT rules:
  dnat_and_snat  external=203.0.113.19   logical=192.168.20.156  -> port d0075095-6232-43a6-b0c0-7fd7a07716a3
  snat           external=203.0.113.41   logical=192.168.20.0/24

Ports:
  d0075095-6232-43a6-b0c0-7fd7a07716a3 ((unnamed))
      Fixed IPs        : 192.168.20.156
      Owner            : server 14d4e13f-fef5-4676-a8e2-a5bdf63c2b6f (new-test2)
      OVN LSP          : type=external, up, addresses=d4:04:e6:4f:7e:cc 192.168.20.156
      HA_Chassis_Group : neutron-5af67e57-7052-49fc-9bad-253275b39986 -> a2172b59-5558-4018-858d-88947e2d9adf (priority=32767, alive, physnets=f20-1-network)