caddy-frontend: support failover backend.

TODO:

• really implement (Luke)
• drop health-check-failover-path, but keep the query string like ?a=b&p= for configured url, https-url. health-check-failover-url and health-check-failover-https-url
• test (add, adapt) (Luke)
• fix post testing (Luke)
• do manual checks on some clusters

/cc @luke @vpelletier