Add OpenConfig DNS provider implementation

[rgildein]

Implement EnsureDNS and DeleteDNS for the OpenConfig provider targeting SR Linux via gNMI. Maps DNSSpec. Domain to /system/dns/config/search and DNSSpec.Servers to /system/dns/servers/server list.
Rejects unsupported fields (adminState: Down, sourceInterfaceName, vrfName) with explicit UnsupportedFieldError.

How I tested it

$ kubectl apply -f - <<EOF
  apiVersion: networking.metal.ironcore.dev/v1alpha1
  kind: DNS
  metadata:
    name: dns
    namespace: default
  spec:
    deviceRef:
      name: leaf1
    domain: example.com
    servers:
      - address: 8.8.8.8
      - address: 1.1.1.1
EOF
dns.networking.metal.ironcore.dev/dns configured
$ gnmic -a 172.20.20.2 --port 57400 -u admin -p 'NokiaSrl1!' --skip-verify --encoding JSON_IETF get --path "openconfig-system:system/dns"
[
  {
    "source": "172.20.20.2",
    "timestamp": 1785331298950342192,
    "time": "2026-07-29T15:21:38.950342192+02:00",
    "updates": [
      {
        "Path": "openconfig-system:system/dns",
        "values": {
          "openconfig-system:system/dns": {
            "config": {
              "search": [
                "example.com"
              ]
            },
            "host-entries": {},
            "servers": {
              "server": [
                {
                  "address": "8.8.8.8",
                  "config": {
                    "address": "8.8.8.8"
                  },
                  "state": {
                    "address": "8.8.8.8"
                  }
                },
                {
                  "address": "1.1.1.1",
                  "config": {
                    "address": "1.1.1.1"
                  },
                  "state": {
                    "address": "1.1.1.1"
                  }
                }
              ]
            },
            "state": {
              "search": [
                "example.com"
              ]
            }
          }
        }
      }
    ]
  }
]

[rgildein]

replaced by: #471