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/config.go

@@ -402,6 +402,8 @@ type HTTPBodyRewriteConfig struct {
 	Type        string
 	Match       string
 	Replacement string
+	// name of the rewriter plugin (via registry)
+	Rewriter string
 }
 
 type NodeFilterConfig struct {