Pull request: all: support more $dnsrewrite rr types

Merge in DNS/adguard-home from 2102-dnsrewrite-2 to master

Updates #2102.
Updates #2452.

Squashed commit of the following:

commit b41e57731b4f276e97202e172fa400067174653d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 22 20:40:56 2020 +0300

    dnsforward: improve naming

commit 70b832ce969d8cdcf4224d221e5e1e2057fba6ee
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 22 19:36:21 2020 +0300

    all: support more $dnsrewrite rr types

internal/dnsforward/dnsrewrite.go

@@ -13,27 +13,55 @@ import (
 )
 
 // filterDNSRewriteResponse handles a single DNS rewrite response entry.
-// It returns the constructed answer resource record.
+// It returns the properly constructed answer resource record.
 func (s *Server) filterDNSRewriteResponse(req *dns.Msg, rr rules.RRType, v rules.RRValue) (ans dns.RR, err error) {
+	// TODO(a.garipov): As more types are added, we will probably want to
+	// use a handler-oriented approach here.  So, think of a way to decouple
+	// the answer generation logic from the Server.
+
 	switch rr {
 	case dns.TypeA, dns.TypeAAAA:
 		ip, ok := v.(net.IP)
 		if !ok {
-			return nil, fmt.Errorf("value has type %T, not net.IP", v)
+			return nil, fmt.Errorf("value for rr type %d has type %T, not net.IP", rr, v)
 		}
 
 		if rr == dns.TypeA {
-			return s.genAAnswer(req, ip.To4()), nil
+			return s.genAnswerA(req, ip.To4()), nil
 		}
 
-		return s.genAAAAAnswer(req, ip), nil
-	case dns.TypeTXT:
+		return s.genAnswerAAAA(req, ip), nil
+	case dns.TypePTR,
+		dns.TypeTXT:
 		str, ok := v.(string)
 		if !ok {
-			return nil, fmt.Errorf("value has type %T, not string", v)
+			return nil, fmt.Errorf("value for rr type %d has type %T, not string", rr, v)
 		}
 
-		return s.genTXTAnswer(req, []string{str}), nil
+		if rr == dns.TypeTXT {
+			return s.genAnswerTXT(req, []string{str}), nil
+		}
+
+		return s.genAnswerPTR(req, str), nil
+	case dns.TypeMX:
+		mx, ok := v.(*rules.DNSMX)
+		if !ok {
+			return nil, fmt.Errorf("value for rr type %d has type %T, not *rules.DNSMX", rr, v)
+		}
+
+		return s.genAnswerMX(req, mx), nil
+	case dns.TypeHTTPS,
+		dns.TypeSVCB:
+		svcb, ok := v.(*rules.DNSSVCB)
+		if !ok {
+			return nil, fmt.Errorf("value for rr type %d has type %T, not *rules.DNSSVCB", rr, v)
+		}
+
+		if rr == dns.TypeHTTPS {
+			return s.genAnswerHTTPS(req, svcb), nil
+		}
+
+		return s.genAnswerSVCB(req, svcb), nil
 	default:
 		log.Debug("don't know how to handle dns rr type %d, skipping", rr)