LBAAS-3995: add project ID to nat gateway

[jvasilevsky]

add project ID to nat gateway

example:

curl -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $DIGITALOCEAN_TOKEN" \
  -d '{
    "name": "test-vpc-nat-gateways",
    "project_id": "8ac2138d-4310-4562-a077-8018e13ab1fb",
    "type": "PUBLIC",
    "region": "tor1",
    "size": 1,
    "vpcs": [
      {
        "vpc_uuid": "0eb1752f-807b-4562-a077-8018e13ab1fb"
      }
    ],
    "udp_timeout_seconds": 30,
    "icmp_timeout_seconds": 30,
    "tcp_timeout_seconds": 30
  }' \
  "https://api.digitalocean.com/v2/vpc_nat_gateways"

Example response:

{
  "vpc_nat_gateway": {
    "id": "70e1b58d-cdec-4e95-b3ee-2d4d95feff51",
    "project_id": "8ac2138d-4310-4562-a077-8018e13ab1fb",
    "name": "test-vpc-nat-gateways",
    "type": "PUBLIC",
    "state": "ACTIVE",
    "region": "tor1",
    "size": 1,
    "vpcs": [
      {
        "vpc_uuid": "0eb1752f-807b-4562-a077-8018e13ab1fb",
        "gateway_ip": "10.118.0.35"
      }
    ],
    "egresses": {
      "public_gateways": [
        {
          "ipv4": "174.138.113.197"
        }
      ]
    },
    "udp_timeout_seconds": 30,
    "icmp_timeout_seconds": 30,
    "tcp_timeout_seconds": 30,
    "created_at": "2025-08-12T18:43:14Z",
    "updated_at": "2025-08-12T19:00:04Z"
  }
}

[SSharma-10]

LGTM