mail/stalwart: New port: Stalwart Mail Server

Stalwart Mail Server is an open-source mail server solution with
JMAP, IMAP4, POP3, and SMTP support and a wide range of modern
features. It is written in Rust and designed to be secure, fast,
robust and scalable.

WWW: https://stalw.art/

PR: 286326
Submitted by: Orville Song <orville@anislet.dev>