plugin/rewrite - extend edns0 local variable support with metadata (#1928)

* - add support of metadata values for edns0 local variables

* - comments from review.

* - simplify label check. Add UT

* - enhance check for Labels, add UT
- remove IsMetadataSet

* - edns0 variable - if variable is not found just ignore the rewrite.

plugin/rewrite/class.go

@@ -1,6 +1,7 @@
 package rewrite
 
 import (
+	"context"
 	"fmt"
 	"strings"
 
@@ -29,7 +30,7 @@ func newClassRule(nextAction string, args ...string) (Rule, error) {
 }
 
 // Rewrite rewrites the the current request.
-func (rule *classRule) Rewrite(state request.Request) Result {
+func (rule *classRule) Rewrite(ctx context.Context, state request.Request) Result {
 	if rule.fromClass > 0 && rule.toClass > 0 {
 		if state.Req.Question[0].Qclass == rule.fromClass {
 			state.Req.Question[0].Qclass = rule.toClass