curl --request GET \
  --url https://api.sumvin.com/v0/auth/siwe/challenge

{
  "_links": {
    "self": {
      "href": "<string>",
      "method": "GET",
      "templated": false,
      "description": "<string>"
    }
  },
  "message": "<string>",
  "nonce": "<string>",
  "expires_at": 123
}