Feature Request: Proxy setting for outgoing HTTP Requests

[alvar-freude]

Thanks for creating Stalwart! I have a feature request which can help to build secure and confidential systems:
As far as I can see, there is no way to set up an HTTP proxy for outgoing HTTP requests (e.g., TLS certificate renewal, spam filter updates, etc.). In my setups, external access is usually only possible via a proxy, which filters traffic based on whitelists for security (and privacy) reasons. Of course, outgoing SMTP must be open, but HTTP (and everything else) is not necessary.

All outgoing HTTP requests should be properly documented too (with an overview in one place), and if feasible there should be a config option to use a local file instead.

This feature request is different from #2643, which requests proxied SMTP connections. Support for HTTP proxies should be easy to implement using the methods provided by the underlying HTTP(S) library.