Request: Add built-in proxy support for IMAP connections

[JFuqX]

Hello,

I'm trying to use imap-tools with HTTP/HTTPS proxies in a multi-threaded environment. Currently, the workaround is using PySocks with global socket modifications, which isn't thread-safe and causes issues when multiple connections try to use different proxies.

Proposed solution:

1. Add proxy support directly to MailBox class
2. Support HTTP/HTTPS/SOCKS proxies via PySocks
3. Allow per-connection proxy configuration

Example desired usage:

mailbox = MailBox(
    "imap.gmail.com",
    proxy_type="http",
    proxy_host="proxy.example.com",
    proxy_port=8080,
    proxy_username="user",
    proxy_password="pass"
).login(email, password)