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

config/parsing/service/parse.go

@@ -10,8 +10,8 @@ import (
 	"github.com/go-gost/core/chain"
 	"github.com/go-gost/core/handler"
 	"github.com/go-gost/core/hop"
-	"github.com/go-gost/core/rewriter"
 	"github.com/go-gost/core/listener"
+	"github.com/go-gost/core/rewriter"
 	"github.com/go-gost/core/logger"
 	"github.com/go-gost/core/observer/stats"
 	"github.com/go-gost/core/recorder"
@@ -356,6 +356,9 @@ func ParseService(cfg *config.ServiceConfig) (service.Service, error) {
 
 	var rew rewriter.Rewriter
 	if cfg.Rewriter != "" {
+		if !registry.RewriterRegistry().IsRegistered(cfg.Rewriter) {
+			serviceLogger.Warnf("rewriter %q not found in registry", cfg.Rewriter)
+		}
 		rew = registry.RewriterRegistry().Get(cfg.Rewriter)
 	}