planner: Add subquery variables (#21032)

[ti-chi-bot]

This is an automated cherry-pick of #21032

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What is changed, added or deleted? (Required)

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• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
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• v6.1 (TiDB 6.1 versions)
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What is the related PR or file link(s)?

pingcap/tidb#63204
pingcap/tidb#63416

• This PR is translated from: planner: Add subquery variables docs#21943
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

/approve

[ti-chi-bot]

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[Oreoxmt]

/unhold