[Feature request] auto configure DKIM key via libdns

# Use case

Since domain access to a domain provider might be already given with an api token for tls-dns challenge, why not support configuring DKIM key automatically using libdns?

- [x] I'm willing to help with the implementation

Start by reading how the existing TLS-DNS challenge integration uses domain-provider API tokens, then investigate how DKIM keys are currently configured and where libdns could fit. Done means defining and implementing automatic DKIM configuration through libdns with a clear validation path and tests.

Written by the indexing model from the issue text.