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/rewriter/parse.go

@@ -39,6 +39,7 @@ func ParseRewriter(cfg *config.RewriterConfig) rewriter.Rewriter {
 				cfg.Name, cfg.Plugin.Addr,
 				plugin.TokenOption(cfg.Plugin.Token),
 				plugin.TLSConfigOption(tlsCfg),
+				plugin.TimeoutOption(cfg.Plugin.Timeout),
 			)
 		}
 	}