caddy-frontend: support failover backend.
TODO:
- really implement (Luke)
-
drop
health-check-failover-path, but keep the query string like?a=b&p=for configuredurl,https-url.health-check-failover-urlandhealth-check-failover-https-url - test (add, adapt) (Luke)
- fix post testing (Luke)
- do manual checks on some clusters
/cc @luke @vpelletier