Let top-level projection can push down to TiFlash when mpp is not enforced.

[LittleFall]

Enhancement

#25450 support top-level projection push down to TiFlash, but it only takes effect when mpp mode is enforced. This changes the semantics of tidb_enforce_mpp: It will introduce new optimization rules instead of just ignoring the optimizer cost estimate.、

It is actually maintaining two sets of optimizers.

#26207 wants to fix it, but it finally closed because the influence on optimizer cost model can't be evaluated.

We need a better way to merge this optimize rule into the current optimizer.