feat(rewriter): implement plugin-based Rewriter module with gRPC and HTTP backends

Add the first implementation of core/rewriter.Rewriter as a plugin-only
component following the bypass/admission pattern (single-value, no
RewriterObject). Includes:

- plugin/rewriter/proto/: protobuf definition with Rewrite RPC returning
  transformed data in the reply
- x/rewriter/plugin/: gRPC and HTTP plugin clients
- x/registry/rewriter.go: hot-reload-safe RewriterRegistry with wrapper
- x/config/config.go: RewriterConfig, Config.Rewriters, ServiceConfig.Rewriter
- x/config/parsing/rewriter/parse.go: config parser (plugin backends only)
- core/handler/option.go: Rewriter field + RewriterOption on handler.Options
- x/config/loader/loader.go: rewriter registration during startup
- x/config/parsing/service/parse.go: inject rewriter into handler options

config/parsing/service/parse.go

@@ -10,6 +10,7 @@ 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/logger"
 	"github.com/go-gost/core/observer/stats"
@@ -353,6 +354,11 @@ func ParseService(cfg *config.ServiceConfig) (service.Service, error) {
 		})
 	}
 
+	var rew rewriter.Rewriter
+	if cfg.Rewriter != "" {
+		rew = registry.RewriterRegistry().Get(cfg.Rewriter)
+	}
+
 	routerOpts = []chain.RouterOption{
 		chain.RetriesRouterOption(cfg.Handler.Retries),
 		chain.TimeoutRouterOption(dialTimeout),
@@ -382,6 +388,7 @@ func ParseService(cfg *config.ServiceConfig) (service.Service, error) {
 			handler.TrafficLimiterOption(registry.TrafficLimiterRegistry().Get(cfg.Handler.Limiter)),
 			handler.ObserverOption(registry.ObserverRegistry().Get(cfg.Handler.Observer)),
 			handler.RecordersOption(recorders...),
+			handler.RewriterOption(rew),
 			handler.LoggerOption(handlerLogger),
 			handler.ServiceOption(cfg.Name),
 			handler.NetnsOption(netnsIn),