Support custom wireguard options

Problem Description

I wish to use custom WG configuration, instead of hard-coded values in the python, I read https://github.com/mitmproxy/mitmproxy/pull/5562#pullrequestreview-1099934509 and I understand the desire to not have lots of code to parse existing configs.

Proposal

Add additional options to the wireguard.conf file. The pub/priv keys are already stored here, and other variables could also be stored here too, such is IPs, listen ports for both instance/peer/dns and in particular, endpoint.

The file could easily be populated by default with the static config already there, which would provide customisation at no expense to those who do not wish to manually enter anything.

Alternatives

There is not really one that requires less effort, but the alternative would be to allow the use of standard wireguard config formats.

Additional context

In my case I have an LTE router at another location and I want to connect mitmproxy via my existing wireguard setup, which uses a FQDN as a means of conveying the current IP, due to having a dynamic one so that I can proxy the webUIs available there over the tunnel.

It would also be nice one day to reintroduce the multi-peer support so one instance of MITM can serve multiple remote locations or devices without having to share a common WG tunnel.

Start by reviewing the wireguard.conf proposal and the referenced pull request review at #5562. Determine how custom IPs, listen ports, DNS settings, endpoints, and existing keys should be represented, and define what configuration behavior and compatibility should count as done.