server: add endpoint which returns all regions in the cluster

Jesse Seldess (jseldess) commented:

PR: https://github.com/cockroachdb/cockroach/pull/67098

From release notes:
> Introduced a `/_status/regions` endpoint which returns all regions along with their availability zones. [#67098][#67098] {% comment %}doc{% endcomment %}

Jira Issue: DOC-1407