feat(forwarder): support plugin-based rewriter in HTTP body rewrite rules

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

internal/util/forwarder/sniffer_http.go

@@ -369,7 +369,7 @@ func (h *Sniffer) httpRoundTrip(ctx context.Context, rw, cc io.ReadWriteCloser,
 
 	// Rewrite request body before wrapping for recording,
 	// so the recorder sees the rewritten content.
-	if err = rewriteReqBody(req, reqBodyRewrites...); err != nil {
+	if err = rewriteReqBody(ctx, req, reqBodyRewrites...); err != nil {
 		log.Errorf("rewrite request body: %v", err)
 		return
 	}
@@ -446,7 +446,7 @@ func (h *Sniffer) httpRoundTrip(ctx context.Context, rw, cc io.ReadWriteCloser,
 		resp.Header.Set("Connection", "close")
 	}
 
-	if err = rewriteRespBody(resp, respBodyRewrites...); err != nil {
+	if err = rewriteRespBody(ctx, resp, respBodyRewrites...); err != nil {
 		log.Errorf("rewrite body: %v", err)
 		return
 	}