Now proxy binary accepts `--auth-backend` CLI option, which determines
auth scheme and cluster routing method. Following backends are currently
implemented:
* legacy
old method, when username ends with `@zenith` it uses md5 auth dbname as
the cluster name; otherwise, it sends a login link and waits for the console
to call back
* console
new SCRAM-based console API; uses SNI info to select the destination
cluster
* postgres
uses postgres to select auth secrets of existing roles. Useful for local
testing
* link
sends login link for all usernames
