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

rewriter/plugin/grpc.go

@@ -0,0 +1,89 @@
+package rewriter
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+
+	"github.com/go-gost/core/logger"
+	"github.com/go-gost/core/rewriter"
+	"github.com/go-gost/plugin/rewriter/proto"
+	"github.com/go-gost/x/internal/plugin"
+	"google.golang.org/grpc"
+)
+
+type grpcPlugin struct {
+	conn   grpc.ClientConnInterface
+	client proto.RewriterClient
+	log    logger.Logger
+}
+
+// NewGRPCPlugin creates a Rewriter plugin based on gRPC.
+func NewGRPCPlugin(name string, addr string, opts ...plugin.Option) rewriter.Rewriter {
+	var options plugin.Options
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	log := logger.Default().WithFields(map[string]any{
+		"kind":     "rewriter",
+		"rewriter": name,
+	})
+	conn, err := plugin.NewGRPCConn(addr, &options)
+	if err != nil {
+		log.Error(err)
+	}
+
+	p := &grpcPlugin{
+		conn: conn,
+		log:  log,
+	}
+	if conn != nil {
+		p.client = proto.NewRewriterClient(conn)
+	}
+	return p
+}
+
+func (p *grpcPlugin) Rewrite(ctx context.Context, b []byte, opts ...rewriter.RewriteOption) ([]byte, error) {
+	if p.client == nil {
+		return b, nil
+	}
+
+	var options rewriter.RewriteOptions
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	md, err := json.Marshal(options.Metadata)
+	if err != nil {
+		return b, err
+	}
+
+	reply, err := p.client.Rewrite(ctx,
+		&proto.RewriteRequest{
+			Data:     b,
+			Metadata: md,
+		})
+	if err != nil {
+		p.log.Error(err)
+		return b, err
+	}
+	if reply == nil || !reply.Ok {
+		return b, nil
+	}
+	if reply.Data != nil {
+		return reply.Data, nil
+	}
+	return b, nil
+}
+
+func (p *grpcPlugin) Close() error {
+	if p.conn == nil {
+		return nil
+	}
+
+	if closer, ok := p.conn.(io.Closer); ok {
+		return closer.Close()
+	}
+	return nil
+}