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/rewriter.go

@@ -0,0 +1,43 @@
+package registry
+
+import (
+	"context"
+
+	"github.com/go-gost/core/rewriter"
+)
+
+// rewriterRegistry implements a hot-reload-safe registry for rewriter.Rewriter.
+type rewriterRegistry struct {
+	registry[rewriter.Rewriter]
+}
+
+// Register stores a Rewriter under the given name.
+func (r *rewriterRegistry) Register(name string, v rewriter.Rewriter) error {
+	return r.registry.Register(name, v)
+}
+
+// Get returns a wrapper that delegates to the currently registered Rewriter.
+// Returns nil if name is empty.
+func (r *rewriterRegistry) Get(name string) rewriter.Rewriter {
+	if name != "" {
+		return &rewriterWrapper{name: name, r: r}
+	}
+	return nil
+}
+
+func (r *rewriterRegistry) get(name string) rewriter.Rewriter {
+	return r.registry.Get(name)
+}
+
+type rewriterWrapper struct {
+	name string
+	r    *rewriterRegistry
+}
+
+func (w *rewriterWrapper) Rewrite(ctx context.Context, b []byte, opts ...rewriter.RewriteOption) ([]byte, error) {
+	v := w.r.get(w.name)
+	if v == nil {
+		return b, nil
+	}
+	return v.Rewrite(ctx, b, opts...)
+}