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

internal/dnsforward/http_test.go

@@ -72,6 +72,7 @@ func TestDNSForwardHTTP_handleGetConfig(t *testing.T) {
 			ProtectionEnabled: true,
 			BlockingMode:      BlockingModeDefault,
 			UpstreamDNS:       []string{"8.8.8.8:53", "8.8.4.4:53"},
+			FallbackDNS:       []string{"9.9.9.10"},
 			EDNSClientSubnet:  &EDNSClientSubnet{Enabled: false},
 		},
 		ConfigModified: func() {},
@@ -225,6 +226,9 @@ func TestDNSForwardHTTP_handleSetConfig(t *testing.T) {
 	}, {
 		name:    "local_ptr_upstreams_null",
 		wantSet: "",
+	}, {
+		name:    "fallbacks",
+		wantSet: "",
 	}}
 
 	var data map[string]struct {