[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_".
* [proxy] Propagate most errors to user
This change enables propagation of most errors to the user
(e.g. auth and connectivity errors). Some of them will be
stripped of sensitive information.
As a side effect, most occurrences of `anyhow::Error` were
replaced with concrete error types.
* [proxy] Box weighty errors
