privilege/privileges: refactor the data struct for user privilege data

[CbcWestwolf]

What problem does this PR solve?

Issue Number: ref #61706

Problem Summary:

What changed and how does it work?

Pick #58945

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.

None

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[codecov]

Codecov Report

Attention: Patch coverage is 64.68750% with 113 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-8.5-20250606-v8.5.2@f43a133). Learn more about missing BASE report.

Additional details and impacted files

@@                       Coverage Diff                        @@
##             release-8.5-20250606-v8.5.2     #61719   +/-   ##
================================================================
  Coverage                               ?   57.5547%           
================================================================
  Files                                  ?       1770           
  Lines                                  ?     629118           
  Branches                               ?          0           
================================================================
  Hits                                   ?     362087           
  Misses                                 ?     242467           
  Partials                               ?      24564           

Flag	Coverage Δ
integration	38.0379% <62.8125%> (?)

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

Components	Coverage Δ
dumpling	52.9278% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	63.1413% <0.0000%> (?)

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[ti-chi-bot]

@CbcWestwolf: The following tests 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-unit-test-ddlv1	d59d172	link	true	/test pull-unit-test-ddlv1
idc-jenkins-ci-tidb/unit-test	d59d172	link	true	/test unit-test

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