Another attempt at #29550, which was reverted. Fallback is not happening
if cert provisioning fails _despite_ having the correct header. But with
the changes in this PR, since we'll listen on `:80`, fallback _will_
happen when cert provisioning fails due to incorrect domain
configuration.
We're also adding [Hurl](https://hurl.dev) based tests. They're not run
in any CI yet. That'll come in soon.
