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"
              ]
            }
          }
        }
      }
    ]
  }
]

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/ironcore-dev/network-operator/internal/provider/openconfig	0.00% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/ironcore-dev/network-operator/internal/provider/openconfig/dns.go	0.00% (ø)	23 (+23)	0	23 (+23)

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.