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