mail proxy: allow dynamically deciding whether to use SMTP authentication to the backend

Describe the feature you'd like to add to nginx

Currently, when using the mail proxy module, the proxy_smtp_auth setting (default: off) determines whether nginx will authenticate itself against the backend SMTP server.

The HTTP authentication server determines which backend to connect to on a per-request basis. It would be nice if it were possible to also determine whether to authenticate against that backend on a per-request (or per-backend) basis.

Describe the problem this feature solves

When using the nginx mail proxy functionality to proxy email traffic over multiple backends, this feature would allow mixing backends that require authentication with backends that don't allow it.