text: Title: [Feature Request] DynDNS API can update subdomains
  Body: **Feature Description:**
  
  Using the [dynDNS API] it is possible to update `A` and `AAAA` records of sub-domains, e.g. `sub.main.dedyn.io`.
  
  One possible way to achieve this would be via the already existing `hostname` parameter in the API:
  
  ```
  curl -u main.dedyn.io:<token> https://update.dedyn.io/?hostname=sub.main.dedyn.io
  ```
  
  **Context:**
  * according to [this comment](https://github.com/desec-io/desec-stack/issues/411#issuecomment-640259255) the current behavior does_not_ allow to do so
  * according to [this comment](https://github.com/desec-io/desec-stack/issues/411#issuecomment-640806623) the required changes _could be a relatively easy change_.
  * this feature request stems from the issue #411.
  
  
  [dynDNS API]: https://desec.readthedocs.io/en/latest/dyndns/update-api.html 