curl --request GET \
  --url https://sandbox.api.btdx.io/external-reference-sources \
  --header 'authorization: <authorization>'

{
  "_meta": {
    "_count": 123,
    "_page": {
      "_pageNumber": 123,
      "_pageSize": 123,
      "_totalElements": 123,
      "_totalPages": 123
    }
  },
  "externalReferenceSources": [
    {
      "code": "<string>",
      "description": "<string>"
    }
  ]
}