Add alternate option to forward plugin (#6681)

Allows the forward plugin to execute the next plugin based on the return code. Similar to the externally mainted alternate plugin https://github.com/coredns/alternate

Based on the idea of chrisohaver@ in #6549 (comment)
Also incoperated the request to rename `alternate` to `next` as an option

I am having issues adding a proper test for functionality. Primarily, I do not know the code base enough and having multiple `dnstest.NewServer` with ResponseWriter does not work. From my testing these are "Singletons'' and only the last defined response writer is used for all servers

Signed-off-by: Jasper Bernhardt <jasper.bernhardt@live.de>

plugin/forward/forward.go

@@ -43,6 +43,8 @@ type Forward struct {
 	from    string
 	ignored []string
 
+	nextAlternateRcodes []int
+
 	tlsConfig     *tls.Config
 	tlsServerName string
 	maxfails      uint32
@@ -194,6 +196,15 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 			return 0, nil
 		}
 
+		// Check if we have an alternate Rcode defined, check if we match on the code
+		for _, alternateRcode := range f.nextAlternateRcodes {
+			if alternateRcode == ret.Rcode && f.Next != nil { // In case we do not have a Next handler, just continue normally
+				if _, ok := f.Next.(*Forward); ok { // Only continue if the next forwarder is also a Forworder
+					return plugin.NextOrFailure(f.Name(), f.Next, ctx, w, r)
+				}
+			}
+		}
+
 		w.WriteMsg(ret)
 		return 0, nil
 	}