Closes #3437. Squashed commit of the following: commit fc4207a6ee1a09ade9db5eb5c8b58f88011db2f9 Author: Ainar Garipov <A.Garipov@AdGuard.COM> Date: Thu Aug 12 18:22:31 2021 +0300 dnsforward: imp code, docs commit 0c608e0b7ca0b68b7810fc1ca798fb7d80d6ac24 Author: Ainar Garipov <A.Garipov@AdGuard.COM> Date: Thu Aug 12 18:01:22 2021 +0300 dnsforward: fix clientid check