autoconfig support

Configuring MUAs is made much easier with support for automatic configuration. Mozilla's de-facto standard for this is quite widely used: https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration

I'm currently manually maintaining one of these at https://autoconfig.ur.gs/.well-known/autoconfig/mail/config-v1.1.xml

There are DNS-based standards too, but I don't think they're as widely used.

We'll need a HTTP(S) listener for automatic TLS, activesync, webmail, etc, support too, so I don't think it's a big deal to add one.

Start with Mozilla's autoconfiguration standard and compare it with the linked example at autoconfig.ur.gs. The issue names no repository files, tests, or entry points, so first locate maddy's existing HTTP(S) listener and configuration paths. Done should include a defined implementation scope and working automatic mail-client configuration support.

Written by the indexing model from the issue text.