[Refactor](Nereids) Simplify get input and output slots for plan/expression.

[wangshuo128]

Proposed changes

Simplify the code of getting input/output slots from Expression or Plan.

new interfaces add

Expression:
getInputSlots: Get all the input slots of the expression.

Plan:

• getOutputSet: Get the output slot set of the plan.
• getInputSlots: Get the input slot set of the plan.

changed interface

TreeNode:

• collect: return set as result instead of list.

Problem summary

Describe your changes.

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[morrySnow]

LGTM