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
* `cloud::legacy` talks to Cloud API V1.
* `cloud::api` defines Cloud API v2.
* `cloud::local` mocks the Cloud API V2 using a local postgres instance.
* It's possible to choose between API versions using the `--api-version` flag.
When failpoint feature is disabled it throws away passed code so code
inside is not guaranteed to compile when feature is disabled. In this
particular case code is obsolete so removing it.
* [proxy] Add SCRAM auth
* [proxy] Implement some tests for SCRAM
* Refactoring + test fixes
* Hide SCRAM mechanism behind `#[cfg(test)]`
Currently we only use it in tests, so we hide all relevant
module behind `#[cfg(test)]` to prevent "unused item" warnings.
