[enhancement](Nereids)print slot name in explain string

[morrySnow]

Proposed changes

Currently, explain string print all expression as slot id, e.g. <slot 1>.
This PR, print its name with slot id instead, e.g. column_a[#1]. For details:

• print qualified table name for OlapScanNode
• print NamedExpression name with SlotId instead of just SlotId
• OlapScanNode's node name use "OlapScanNode" instead of table name

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...

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.