Pull request 1992: 3701-fallback-dns-check

Squashed commit of the following:

commit a5191def0b950cb7ab1ea737e9c8c6e8b65ec720
Merge: 7d127bfca 5bd452516
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Wed Sep 6 12:21:08 2023 +0300

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

commit 7d127bfca54ac940c611681e6f48b89881491602
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Sep 5 13:21:35 2023 +0300

    client: add fallback_dns to the upstream test

commit 970da7215503b5f05c53fcad47c715a96d9fc3c4
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Sep 4 19:23:35 2023 +0300

    all: add fallback dns check

openapi/CHANGELOG.md

@@ -6,6 +6,11 @@
 
 ## v0.107.37: API changes
 
+### The new field `"fallback_dns"` in `UpstreamsConfig` object
+
+* The new field `"fallback_dns"` in `POST /control/test_upstream_dns` is the
+  list of fallback DNS servers to test.
+
 ### The new field `"fallback_dns"` in `DNSConfig` object
 
 * The new field `"fallback_dns"` in `GET /control/dns_info` and `POST

openapi/openapi.yaml

@@ -1503,7 +1503,7 @@
           - 'tls://1.0.0.1'
     'UpstreamsConfig':
       'type': 'object'
-      'description': 'Upstreams configuration'
+      'description': 'Upstream configuration to be tested'
       'required':
       - 'bootstrap_dns'
       - 'upstream_dns'
@@ -1511,8 +1511,7 @@
         'bootstrap_dns':
           'type': 'array'
           'description': >
-            Bootstrap servers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Bootstrap DNS servers, port is optional after colon.
           'items':
             'type': 'string'
           'example':
@@ -1521,18 +1520,25 @@
         'upstream_dns':
           'type': 'array'
           'description': >
-            Upstream servers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Upstream DNS servers, port is optional after colon.
           'items':
             'type': 'string'
           'example':
           - 'tls://1.1.1.1'
           - 'tls://1.0.0.1'
+        'fallback_dns':
+          'type': 'array'
+          'description': >
+            Fallback DNS servers, port is optional after colon.
+          'items':
+            'type': 'string'
+          'example':
+          - '8.8.8.8'
+          - '1.1.1.1:53'
         'private_upstream':
           'type': 'array'
           'description': >
-            Local PTR resolvers, port is optional after colon.  Empty value will
-            reset it to default values.
+            Local PTR resolvers, port is optional after colon.
           'items':
             'type': 'string'
           'example':