feat(forwarder): support plugin-based rewriter in HTTP body rewrite rules

Each rewriteBody/rewriteResponseBody/rewriteRequestBody rule can now
optionally reference a named rewriter plugin (HTTP/gRPC backend) via
the rewriter field. When set, body rewriting delegates to the plugin
rather than applying pattern/replacement, while content-type filtering
still applies.

Coincident fixes:
- rewriter/plugin/grpc: return nil when conn is nil (avoid nil-ptr panic)
- rewriter/plugin/http: drain response body in defer to prevent leaks
- config/parsing/rewriter: pass TimeoutOption to gRPC plugin
- config/parsing/service: warn when referenced rewriter is not registered

rewriter/plugin/grpc.go

@@ -33,14 +33,15 @@ func NewGRPCPlugin(name string, addr string, opts ...plugin.Option) rewriter.Rew
 	if err != nil {
 		log.Error(err)
 	}
+	if conn == nil {
+		return nil
+	}
 
 	p := &grpcPlugin{
 		conn: conn,
 		log:  log,
 	}
-	if conn != nil {
-		p.client = proto.NewRewriterClient(conn)
-	}
+	p.client = proto.NewRewriterClient(conn)
 	return p
 }