feat: add option to bind outgoing connections to a specific interface

[shahradelahi]

This adds a new interface configuration option to doh-client that allows users to specify a network interface for all outgoing DNS queries (including bootstrap and passthrough traffic).

[m13253]

I super love this feature. Please give me some time to review this.

By the way, does it work correctly on dual-stack networks? I remembr Golang’s HTTP client uses Happy Eyeballs algorithm. (I could be wrong.) Is this new function compatible with it?

[shahradelahi]

I totally forgot about that. You're right, binding to a specific local IP breaks the default Happy Eyeballs behavior. I'll update the PR with a custom DialContext to handle dual-stack properly.

[m13253]

@vinnyperella Sorry to disturb you.
Do you have access to any environment with multiple network cards + IPv4/IPv6 dual stack + an (iptables) firewall that can intermittently drop IPv4/IPv6 packets to test this new feature?
I appreciate it very much!

I am considering to combining this new feature along #186 in the upcoming release but I currently don’t have enough time to test it throughoutly to make sure it doesn’t break anyone’s setup.