Allow TiFlash coprocessor only when `keep_order` is required

## Enhancement

https://docs.pingcap.com/tidb/stable/system-variables/#tidb_allow_tiflash_cop-new-in-v730
In TiDB 7.3.0, the variable `tidb_allow_tiflash_cop` is set to false by default. The original motivation for this change was that the planner might generate TiFlash coprocessor requests for simple table scans (with or without filters) instead of MPP tasks, which could lead to suboptimal performance.

However, coprocessor requests are still necessary when the `keep_order` property is required, as MPP execution cannot preserve row order. Therefore, a better approach might be to allow TiFlash coprocessor requests only when `keep_order` is true. With this behavior, the `tidb_allow_tiflash_cop` variable can remain enabled (true) without risking performance regressions.