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/setup.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/coredns/caddy"
@@ -289,7 +290,22 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 		}
 		f.ErrLimitExceeded = errors.New("concurrent queries exceeded maximum " + c.Val())
 		f.maxConcurrent = int64(n)
+	case "next":
+		args := c.RemainingArgs()
+		if len(args) == 0 {
+			return c.ArgErr()
+		}
 
+		for _, rcode := range args {
+			var rc int
+			var ok bool
+
+			if rc, ok = dns.StringToRcode[strings.ToUpper(rcode)]; !ok {
+				return fmt.Errorf("%s is not a valid rcode", rcode)
+			}
+
+			f.nextAlternateRcodes = append(f.nextAlternateRcodes, rc)
+		}
 	default:
 		return c.Errf("unknown property '%s'", c.Val())
 	}