Pull request: 4865-refactor-dns-handlers

Updates #4865.

Squashed commit of the following:

commit b874088ee72dfd0cae5f4102fac87e1f26245ddf
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Sep 2 14:01:52 2022 +0300

    dnsforward: imp code, docs

commit a1b95fda58b777a54e7dcd57f0c419200f8fdc15
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Sep 1 19:37:45 2022 +0300

    all: refactor dns handler; opt

internal/filtering/safebrowsing.go

@@ -325,12 +325,12 @@ func (d *DNSFilter) checkSafeBrowsing(
 	}
 
 	res = Result{
-		IsFiltered: true,
-		Reason:     FilteredSafeBrowsing,
 		Rules: []*ResultRule{{
 			Text:         "adguard-malware-shavar",
 			FilterListID: SafeBrowsingListID,
 		}},
+		Reason:     FilteredSafeBrowsing,
+		IsFiltered: true,
 	}
 
 	return check(sctx, res, d.safeBrowsingUpstream)
@@ -359,12 +359,12 @@ func (d *DNSFilter) checkParental(
 	}
 
 	res = Result{
-		IsFiltered: true,
-		Reason:     FilteredParental,
 		Rules: []*ResultRule{{
 			Text:         "parental CATEGORY_BLACKLISTED",
 			FilterListID: ParentalListID,
 		}},
+		Reason:     FilteredParental,
+		IsFiltered: true,
 	}
 
 	return check(sctx, res, d.parentalUpstream)