API Gateway allows users to "claim" a domain name for use as a custom
hostname for deployed API endpoints. The domain name resource just claims
the domain name; a user would then use a "base path mapping" resource
(to be implemented in a later commit) to map a particular API to a
particular path prefix on that domain.
The acceptance tests contain some TLS certificates that expire in 2026;
we'll need to generate some more certificates before we get there.
Ninir