External DNS should be configurable

in https://github.com/oxidecomputer/omicron/issues/6944 and #8047 we got external DNS to serving an SOA record, but the fields of that SOA record are basically hardcoded. i mentioned the current hardcoded values in this comment. we really ought to have a few things here:

• the SOA RNAME should be configurable to the email address a rack's operator can receive email at. the rack has no reason to send email to this address, but operators deserve to have reasonable DNS records.
• TTL/REFRESH/RETRY/EXPIRE/MINIMUM for the external zone's SOA record should be configurable. this is presumably more immediately observable: these are fields that DNS caching/replication systems might operate in terms of.
• the delegated domain ought to be configurable. it's currently set at RSS time, we have no mechanism to change it at the moment.

i think SOA fields are just a question of storage and API plumbing. wherever those fields live, it's probably worth asking if it should be with the delegated domain.

the SOA fields we have are definitely not ideal, and i expect it's likely someone will want to configure them sooner than later.

i'm not aware of anyone wanting to change the delegated domain today, but it seems pretty reasonable to want. changing the delegated domain is probably a more complicated separate operation - at the very least one would want to track down what, if any, names get derived from it and would need to be updated.

Start by tracing how external DNS currently serves the SOA record and how the delegated domain is set at RSS time. Identify where the SOA fields and delegated domain are stored and exposed through API plumbing. Done means the SOA RNAME, TTL, REFRESH, RETRY, EXPIRE, MINIMUM, and delegated domain can be configured, with derived names updated safely when the domain changes.