Each rewriteBody/rewriteResponseBody/rewriteRequestBody rule can now
optionally reference a named rewriter plugin (HTTP/gRPC backend) via
the rewriter field. When set, body rewriting delegates to the plugin
rather than applying pattern/replacement, while content-type filtering
still applies.
Coincident fixes:
- rewriter/plugin/grpc: return nil when conn is nil (avoid nil-ptr panic)
- rewriter/plugin/http: drain response body in defer to prevent leaks
- config/parsing/rewriter: pass TimeoutOption to gRPC plugin
- config/parsing/service: warn when referenced rewriter is not registered