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
ginuerzh