Add design document for common table expression

[wjhuang2016]

Development Task

Related PR：#18247 and #19253

But they are not detailed enough, for example, they don't take the ‘MPP’ into consideration.

[tisonkun]

IMO supports for MPP can be considered individually, given that planning and execution model of tidb and those of tiflash(which contains MPP logics) is actually different. We can hardly consider all details up front but prevent blocking further evolving.

[wjhuang2016]

IMO supports for MPP can be considered individually, given that planning and execution model of tidb and those of tiflash(which contains MPP logics) is actually different. We can hardly consider all details up front but prevent blocking further evolving.

No, we do need to consider them as a whole feature. That's why the detailed design is not decided yet.

[tisonkun]

subtask of #17472