Add a new distributed cost model in Nereids. The new cost model models the cost of the pipeline execute engine by dividing cost into run and start costs. They are:
* START COST: the cost from starting to emitting the fist tuple
* RUN COST: the cost from emitting the first tuple to emitting all tuples
For the parent operator and child operator, we assume the timeline of them is:
```
child start ---> child run --------------------> finish
|---> parent start ---> parent run -> finish
```
Therefore, in the parallel model, we can get:
```
start_cost(parent) = start_cost(child) + start_cost(parent)
run_cost(parent) = max(run_cost(child), start_cost(parent) + run_cost(parent))
```
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