Pull request 1975: 3701-fallback-dns-api

Squashed commit of the following:

commit 0f96137c629d7df99d40e479f66dd78248185ca3
Merge: 0d640fe37 aac36a2d2
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Aug 30 12:53:03 2023 +0300

    Merge branch 'master' into 3701-fallback-dns-api

commit 0d640fe37a563d9fcea1ff4b6c86f37629af0ebd
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Aug 30 12:49:05 2023 +0300

    dnsforward: imp tests

commit 6b7f7aad76566b70852146bc6629ae7fb71eec09
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Aug 25 18:51:34 2023 +0300

    client: add fallback dns field

commit c2ac7e3656431f7d44645786296e58ac7d012fed
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Thu Aug 24 13:22:07 2023 +0300

    all: add fallback dns api

client/src/components/Settings/Dns/Upstream/index.js

@@ -10,6 +10,7 @@ const Upstream = () => {
     const dispatch = useDispatch();
     const {
         upstream_dns,
+        fallback_dns,
         bootstrap_dns,
         upstream_mode,
         resolve_clients,
@@ -21,6 +22,7 @@ const Upstream = () => {
 
     const handleSubmit = (values) => {
         const {
+            fallback_dns,
             bootstrap_dns,
             upstream_dns,
             upstream_mode,
@@ -30,6 +32,7 @@ const Upstream = () => {
         } = values;
 
         const dnsConfig = {
+            fallback_dns,
             bootstrap_dns,
             upstream_mode,
             resolve_clients,
@@ -52,6 +55,7 @@ const Upstream = () => {
                 <Form
                     initialValues={{
                         upstream_dns: upstreamDns,
+                        fallback_dns,
                         bootstrap_dns,
                         upstream_mode,
                         resolve_clients,