feat: Why doesn't the limit-req plugin support `rules` like limit-conn and ai-rate-limiting?

### Description

In APISIX 3.16, the `limit-conn` and `ai-rate-limiting` plugins added support for the `rules` feature via PR #13000, which allows conditional rate limiting based on request attributes, but not yet to `limit-req`. It would be great to have this feature in `limit-req` as well to maintain consistency and enable more flexible rate limiting scenarios.