Support for DuckDNS Aliases

Is your feature request related to a problem? Please describe.

Please provide support for DNS aliases on the certificate when doing DNS challenge certificate requests, I’m specifically referring to the DuckDNS option; but I suppose this would be valid for any provider.

Describe the solution you'd like

• Have a DuckDNS domain, example: example-domain.duckdns.org
• Have your own domain which is a CNAME reference to the DuckDNS domain: awesome.example.com -> example-domain.duckdns.org
• In NGinx Proxy Manager, have an option to add additional aliases to the certificate request (potentially by typing/selecting them manually, but would be better if it would automatically pick up the other DNS names from the configuration).
• Certificate is then issued to the DuckDNS names, with alternative names as the additional DNS names.

Describe alternatives you've considered

There is a HomeAssistant Add-On which already does something similar (LetsEncrypt certificates for DuckDNS domains) -> https://github.com/home-assistant/addons/tree/master/duckdns
See original implementation details at: https://github.com/home-assistant/addons/pull/1253

Additional context

The issue names no files, tests, or entry points; begin by locating the certificate request and DuckDNS provider configuration flows. Done means supporting a CNAME alias alongside the DuckDNS name and issuing a certificate whose alternative names include the configured aliases.