planner: add variables for instance plan cache

[qw4990]

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What is the related PR or file link(s)?

• This PR is translated from: planner: add variables for instance plan cache docs-cn#18557
• Other reference link(s): planner: instance/node level plan cache - phase 1 tidb#54057

Do your changes match any of the following descriptions?

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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-19 08:17:02.699120358 +0000 UTC m=+1121892.439544297: ☑️ agreed by Oreoxmt.
• 2024-09-27 09:29:49.302704783 +0000 UTC m=+1544.722917795: ☑️ agreed by qiancai.

[ti-chi-bot]

@qw4990: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	d7184bd	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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