Sqservices 1721 acme be new nonce endpoint

[battermann]

https://wearezeta.atlassian.net/browse/SQSERVICES-1721

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, or feature configs have changed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If configuration options have been added or removed, check docs/developer/adding-config-options and follow the steps there.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If public end-points have been changed or added: does nginz need un upgrade?

[fisx]

LGTM! Except for the CI problem (haven't looked what it's about) and the remaining unresolved comments (which we discussed offline), this is ready.