Question: "bootstrap prefer ipv6"

Hi.
I found this commit [(Pull request 250: 4262-ipv6-pref)](https://github.com/AdguardTeam/dnsproxy/commit/f8f22ab752e825c655c5174becbfce0a4c430fdc) related to bootstrap upstream's strategy.

Looks like it could be configurable, but I didn’t find the parameter related to this in ```main.go```.
I want to ask how to configure this feature in yaml config file.

Thanks for any help.