planner: support ScalarSubQuery to display them in EXPLAIN (#45252)

[ti-chi-bot]

This is an automated cherry-pick of #45252

What problem does this PR solve?

Issue Number: close #22076

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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Adding the do-not-merge/cherry-pick-not-approved label.

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[fixdb]

+1

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: fixdb
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-27 00:17:17.287562784 +0000 UTC m=+399979.882093768: ☑️ agreed by fixdb.

[ti-chi-bot]

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