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

registry/registry.go

@@ -21,6 +21,7 @@ import (
 	reg "github.com/go-gost/core/registry"
 	"github.com/go-gost/core/resolver"
 	"github.com/go-gost/core/router"
+	"github.com/go-gost/core/rewriter"
 	"github.com/go-gost/core/sd"
 	"github.com/go-gost/core/service"
 	"github.com/go-gost/x/limiter/quota"
@@ -46,6 +47,7 @@ var (
 	resolverReg  reg.Registry[resolver.Resolver]   = new(resolverRegistry)
 	hostsReg     reg.Registry[hosts.HostMapper]    = new(hostsRegistry)
 	recorderReg  reg.Registry[recorder.Recorder]   = new(recorderRegistry)
+	rewriterReg  reg.Registry[rewriter.Rewriter]   = new(rewriterRegistry)
 
 	trafficLimiterReg reg.Registry[traffic.TrafficLimiter] = new(trafficLimiterRegistry)
 	connLimiterReg    reg.Registry[conn.ConnLimiter]       = new(connLimiterRegistry)
@@ -187,6 +189,11 @@ func RecorderRegistry() reg.Registry[recorder.Recorder] {
 	return recorderReg
 }
 
+// RewriterRegistry returns the global registry of rewriter instances.
+func RewriterRegistry() reg.Registry[rewriter.Rewriter] {
+	return rewriterReg
+}
+
 // TrafficLimiterRegistry returns the global registry of traffic limiter instances.
 func TrafficLimiterRegistry() reg.Registry[traffic.TrafficLimiter] {
 	return trafficLimiterReg