Run a nameserver for the {amnesia,tails}.boum.org sub-zones

Originally created by @intrigeri on #16232 (Redmine)

… so our mirror admins can keep controlling it once boum.org’s NS has migrated to its new home.

Then:

• merge the 2 mirrors.git somehow
• adapt mirror team scripts if needed
• update mirror team doc
• add monitoring of this NS
• -document on https://tails.boum.org/contribute/working_together/roles/sysadmins/- (tails/tails#16254)

Related issues

• Blocked by tails/sysadmin#15513