plan change from tikv indexLookUp to tiflash full scan with pushed down filters.

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

### 2. What did you expect to see? (Required)

two perspective of this plan:

1: to ensure stability, the CBO should adopt a more conservative approach when costing a TiFlash full scan. Instead of aggressively reducing the estimated row count to make it appear cheaper for pushed down filter;

2: should we must first validate that full scan can efficiently support a required concurrency from the executor layer? or learning some things from executed same plan, and feel the scan pressure and avoid the same plan next time?

this is compared plan from v7.5.3

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)

next-gen and master