Re-provision otp secret key for the invite

Example response

{
  "_links": [
    {
      "href": "https://an.example.link",
      "method": "GET",
      "rel": "self"
    }
  ],
  "email": "example@example.gov.uk",
  "otp_key": "ABC123",
  "role": "view-only",
  "type": "service"
}