Support shared CTE execution on MPP

[winoros]

Feature Request

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

Currently, if we want to execute CTE with MPP enabled, we can only get the plan that the CTE producer is run on TiDB. And thus, the consumer is also run on TiDB. So the performance loss.
We've created a variable that inlining all the CTEs to make the query fully run on MPP.

But it might also cause performance loss since the CTE producer might be very complex. If we inline them all, the complex producer segment will be executed multiple times.

Describe the feature you'd like:

So we need to support the shared CTE execution on MPP side.
Empower TiDB to execute fully on MPP with the producer only running once.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: