planner: add a fix control to allow generate index merge path when normal index lookup path exists (#52872)

[ti-chi-bot]

This is an automated cherry-pick of #52872

What problem does this PR solve?

Issue Number: close #52869

What changed and how does it work?

Simple change. Please see the issue and code.

Check List

Tests

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

  • I checked and no code files have been changed.

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.

如果查询有除了全表扫描以外的单索引扫描方式可以选择，优化器不会自动选择索引合并。现在可以通过 Optimizer Fix Controls 机制解除这个限制。
If the optimizer can choose the single index scan method (other than full table scan) for a query plan, the optimizer will not automatically use index merge. Now this limitation can be disabled by using the Optimizer Fix Controls.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (release-8.1@ac5612e). Click here to learn what that means.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #52931   +/-   ##
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  Coverage               ?   71.5982%           
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  Files                  ?       1463           
  Lines                  ?     423182           
  Branches               ?          0           
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  Hits                   ?     302991           
  Misses                 ?      99846           
  Partials               ?      20345           

Flag	Coverage Δ
unit	71.5982% <100.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9957% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	40.9786% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, time-and-fate

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-28 07:15:00.724694869 +0000 UTC m=+168654.481830441: ☑️ agreed by AilinKid.
• 2024-04-28 07:31:15.770342033 +0000 UTC m=+169629.527477599: ☑️ agreed by time-and-fate.