Pull request: 3136 show default resolvers

Merge in DNS/adguard-home from 3136-show-default to master

Closes #3136.

Squashed commit of the following:

commit add09a772dddcccd404056e7649c2d3350a79fec
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Jun 1 11:11:24 2021 +0300

    openapi: fix typo

commit 69e402c49318d53a0d03b81207644d9fb04a139a
Merge: 76e8492c e17e1f20
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Jun 1 11:09:51 2021 +0300

    Merge branch 'master' into 3136-show-default

commit 76e8492c8da679e204ceb7a77f1c6f73a2488040
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Jun 1 10:55:09 2021 +0300

    client: check upstreams

commit 05fe6ea196a1ed9386eec25dbfbe22779fe7bcfd
Author: Vlad <v.abdulmyanov@adguard.com>
Date:   Mon May 31 19:23:35 2021 +0300

    add defaul resolvers line

commit 8d63c7df9947c9588478d1173834b42569fd8951
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon May 31 11:56:08 2021 +0300

    all: imp changelog

commit e3912e3b20eca9dcf90ddddaa5edb54d1e0cfe6e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri May 28 20:23:05 2021 +0300

    all: add local ptr resolvers addresses output

internal/dnsforward/http.go

@@ -96,12 +96,25 @@ func (s *Server) getDNSConfig() dnsConfig {
 }
 
 func (s *Server) handleGetConfig(w http.ResponseWriter, r *http.Request) {
-	resp := s.getDNSConfig()
+	defLocalPTRUps, err := s.filterOurDNSAddrs(s.sysResolvers.Get())
+	if err != nil {
+		log.Debug("getting dns configuration: %s", err)
+	}
+
+	resp := struct {
+		dnsConfig
+		// DefautLocalPTRUpstreams is used to pass the addresses from
+		// systemResolvers to the front-end.  It's not a pointer to the slice
+		// since there is no need to omit it while decoding from JSON.
+		DefautLocalPTRUpstreams []string `json:"default_local_ptr_upstreams,omitempty"`
+	}{
+		dnsConfig:               s.getDNSConfig(),
+		DefautLocalPTRUpstreams: defLocalPTRUps,
+	}
 
 	w.Header().Set("Content-Type", "application/json")
 
-	enc := json.NewEncoder(w)
-	if err := enc.Encode(resp); err != nil {
+	if err = json.NewEncoder(w).Encode(resp); err != nil {
 		httpError(r, w, http.StatusInternalServerError, "json.Encoder: %s", err)
 		return
 	}