feat: save DNS information in Domain Status

[DrPsychick]

Suggestion

As suggested in #29, the DNS configuration of Domains could be saved to Status in the resource, so that it is accessible in k8s for later automation, i.e. DNS record checks or updates.

• Extend the CRD to include DNS records in Status
• Add a ready condition for DNS records
• When fetching a Domain from the API, check for DNS records and save them to Status of the resource
• Support creation of DNS records in MailU through an optional flag (in Spec?)