support DVSNI domain name validation for Let's Encrypt / ACME

[dpocock]

It would be useful if SEMS could natively handle DVSNI requests for Let's Encrypt certificates.

A benefit of this approach is that the SEMS process doesn't have to be restarted during certificate setup / renewal and the user doesn't have to manually do anything like DNS TXT records for validation.

Spec:
https://letsencrypt.github.io/acme-spec/#rfc.section.7

Discussion about adding this feature in reSIProcate:
https://www.resiprocate.org/bugzilla/show_bug.cgi?id=131