Support CTE in TiFlash

[xzhangxian1008]

Feature Request

Is your feature request related to a problem? Please describe:

Sqls with cte can not be pushed down to the tiflash.

Describe the feature you'd like:

I'd like to push the sql with cte down to the tiflash so that we can accelerate the sql processing.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[xzhangxian1008]

/assign