Custom SOCKS5 0x80 proxy authentication for IMAPS access

**Is your feature request related to a problem? Please describe.**
My problem is that I have a very specific SOCKS5 proxy to retrieve my emails through. This proxy only accepts a [0x80 / X'80'](https://www.ietf.org/rfc/rfc1928.txt) authentication with a JWT token. Unfortunately the SOCKS5 proxy which is built into the package only allows basic authentication.
For non-SSL SMTP access I was able to create a workaround using my own Socket implementation and the `com.sun.mail.smtp.SMTPTransport`. For IMAPS my problem is that I can't just put a custom Socket in there since I a. don't want to mess with the security layer and b. haven't even found a workaround to do this so far.

The resulting problem for me is: I cannot access my IMAPS Server.

**Describe the solution you'd like**
A custom authentication callback for the SOCKS5 proxy being used, so I can implement the proxy authentication myself.

**Describe alternatives you've considered**
Creating an SSLSocket from my existing custom socket implementation but this poses two problems:

1. I wasn't able to get an SSLSocket from my custom Socket yet
2. I still don't know how I would use that SSLSocket to retrieve emails from the IMAP server

**Additional context**

Start by locating the package's built-in SOCKS5 authentication and the IMAPS connection entry point, then review how a custom authentication callback could be exposed without bypassing TLS. Define what the 0x80/JWT exchange must receive and return, and verify IMAPS can use it; the issue names no files or tests, so repository exploration is required.

Written by the indexing model from the issue text.