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/http.go

@@ -0,0 +1,104 @@
+package rewriter
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http"
+
+	"github.com/go-gost/core/logger"
+	"github.com/go-gost/core/rewriter"
+	"github.com/go-gost/x/internal/plugin"
+)
+
+type httpPluginRequest struct {
+	Data     []byte `json:"data"`
+	Metadata []byte `json:"metadata"`
+}
+
+type httpPluginResponse struct {
+	OK   bool   `json:"ok"`
+	Data []byte `json:"data"`
+}
+
+type httpPlugin struct {
+	url    string
+	client *http.Client
+	header http.Header
+	log    logger.Logger
+}
+
+// NewHTTPPlugin creates a Rewriter plugin based on HTTP.
+func NewHTTPPlugin(name string, url string, opts ...plugin.Option) rewriter.Rewriter {
+	var options plugin.Options
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	return &httpPlugin{
+		url:    url,
+		client: plugin.NewHTTPClient(&options),
+		header: options.Header,
+		log: logger.Default().WithFields(map[string]any{
+			"kind":     "rewriter",
+			"rewriter": name,
+		}),
+	}
+}
+
+func (p *httpPlugin) 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
+	}
+
+	rb := httpPluginRequest{
+		Data:     b,
+		Metadata: md,
+	}
+	v, err := json.Marshal(&rb)
+	if err != nil {
+		return b, err
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, p.url, bytes.NewReader(v))
+	if err != nil {
+		return b, err
+	}
+
+	if p.header != nil {
+		req.Header = p.header.Clone()
+	}
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := p.client.Do(req)
+	if err != nil {
+		return b, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return b, nil
+	}
+
+	res := httpPluginResponse{}
+	if err := json.NewDecoder(resp.Body).Decode(&res); err != nil {
+		return b, err
+	}
+
+	if !res.OK {
+		return b, nil
+	}
+	if res.Data != nil {
+		return res.Data, nil
+	}
+	return b, nil
+}