Assign phone number to campaign

curl --request POST \
  --url https://api.zavu.dev/v1/10dlc/campaigns/{campaignId}/phone-numbers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "phoneNumberId": "pn_abc123"
}
'

{
  "assignment": {
    "id": "<string>",
    "campaignId": "<string>",
    "phoneNumberId": "<string>",
    "status": "pending",
    "createdAt": "2023-11-07T05:31:56Z",
    "updatedAt": "2023-11-07T05:31:56Z",
    "failureReason": "<string>",
    "assignedAt": "2023-11-07T05:31:56Z"
  }
}