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

openapi/openapi.yaml

@@ -1432,6 +1432,16 @@
           'example':
           - 'tls://1.1.1.1'
           - 'tls://1.0.0.1'
+        'fallback_dns':
+          'type': 'array'
+          'description': >
+            List of fallback DNS servers used when upstream DNS servers are not
+            responding.  Empty value will clear the list.
+          'items':
+            'type': 'string'
+          'example':
+          - '8.8.8.8'
+          - '1.1.1.1:53'
         'upstream_dns_file':
           'type': 'string'
         'protection_enabled':