Problem Description

This has come up a few times now: mitmproxy currently just opens asyncio sockets with the unresolved hostnames. Python's stdlib will then use libc's getaddrinfo, which always includes /etc/hosts. This can be very annoying, see e.g. #6737. We want a way to teach mitmproxy to use a custom DNS server.

Proposal

1. Implement an async getaddrinfo method that resolves a hostname using a custom DNS server. This could be done in either Python or Rust in mitmproxy-rs. My first best guess would be to go with https://docs.rs/hickory-resolver/latest/hickory_resolver/ in Rust.
2. Write an addon that defines a dns_server option and monkeypatches/replaces the current event loop's getaddrinfo to use our custom implementation if dns_server is set.

Ideally we start with a PR that does just the first part + tests before getting to part two.