API: Allow opting out of default NS records at domain creation

[OdyX]

Currently, Domain.update_delegation() enforces the DEFAULT_NS settings, which is currently set at ["ns1.desec.io.","ns2.desec.org."].

(I note that it doesn't respect the DEFAULT_NS_TTL settings, but that's for a different bug).

In the context of the introduction of alternative NS names, it would be nice for these NS records to be either:

• configurable at creation time

  API consumers (OpenTofu/Terraform, etc) could then configure the NS records they want by updating the "domain object"

• optional

  API consumers (OpenTofu/Terraform, etc) would then need to configure the NS records as a separate step

In terms of "separation of concern", I find the latter option better; this way all the records that one has in a zone (and which are visible in the web interface) are all separate records, known as such from the start:

• create the domain
• add records
• update records

[peterthomassen]

We'll consider this once the new nameserver hostnames are no longer in experimental phase. This is expected to take a few months, partly as we're trying to resolve the incompatibility with .de.