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/config.go

@@ -316,6 +316,11 @@ type RecorderObject struct {
 	Metadata map[string]any `yaml:",omitempty" json:"metadata,omitempty"`
 }
 
+type RewriterConfig struct {
+	Name   string        `json:"name"`
+	Plugin *PluginConfig `yaml:",omitempty" json:"plugin,omitempty"`
+}
+
 type LimiterConfig struct {
 	Name   string        `json:"name"`
 	Limits []string      `yaml:",omitempty" json:"limits,omitempty"`
@@ -487,8 +492,9 @@ type ServiceConfig struct {
 	Logger     string            `yaml:",omitempty" json:"logger,omitempty"`
 	Loggers    []string          `yaml:",omitempty" json:"loggers,omitempty"`
 	Observer   string            `yaml:",omitempty" json:"observer,omitempty"`
-	Recorders  []*RecorderObject `yaml:",omitempty" json:"recorders,omitempty"`
-	Handler    *HandlerConfig    `yaml:",omitempty" json:"handler,omitempty"`
+	Rewriter  string            `yaml:",omitempty" json:"rewriter,omitempty"`
+	Recorders []*RecorderObject `yaml:",omitempty" json:"recorders,omitempty"`
+	Handler   *HandlerConfig    `yaml:",omitempty" json:"handler,omitempty"`
 	Listener   *ListenerConfig   `yaml:",omitempty" json:"listener,omitempty"`
 	Forwarder  *ForwarderConfig  `yaml:",omitempty" json:"forwarder,omitempty"`
 	Metadata   map[string]any    `yaml:",omitempty" json:"metadata,omitempty"`
@@ -637,6 +643,7 @@ type Config struct {
 	Routers    []*RouterConfig    `yaml:",omitempty" json:"routers,omitempty"`
 	SDs        []*SDConfig        `yaml:"sds,omitempty" json:"sds,omitempty"`
 	Recorders  []*RecorderConfig  `yaml:",omitempty" json:"recorders,omitempty"`
+	Rewriters  []*RewriterConfig  `yaml:",omitempty" json:"rewriters,omitempty"`
 	Limiters   []*LimiterConfig   `yaml:",omitempty" json:"limiters,omitempty"`
 	Quotas     []*QuotaConfig     `yaml:",omitempty" json:"quotas,omitempty"`
 	CLimiters  []*LimiterConfig   `yaml:"climiters,omitempty" json:"climiters,omitempty"`