[proxy] Add the `password hack` authentication flow
This lets us authenticate users which can use neither
SNI (due to old libpq) nor connection string `options`
(due to restrictions in other client libraries).
Note: `PasswordHack` will accept passwords which are not
encoded in base64 via the "password" field. The assumption
is that most user passwords will be valid utf-8 strings,
and the rest may still be passed via "password_".
It was the only test that used the 'schema' argument to the connect()
function. I'm about to refactor the option handling and will remove
the special 'schema' argument altogether, so rewrite the test to not
use it.
