docs: design for extension auth plugin

[yzhan1]

What problem does this PR solve?

Issue Number: ref #53181

Problem Summary:

This is the design doc for #53181.

What changed and how does it work?

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

[ti-chi-bot]

Hi @yzhan1. Thanks for your PR.

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[tiprow]

Hi @yzhan1. Thanks for your PR.

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[CbcWestwolf]

LGTM

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[lcwangchao]

A comment about compatibility. Rest LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CbcWestwolf, lcwangchao

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-13 15:49:17.262349122 +0000 UTC m=+1495511.019484700: ☑️ agreed by CbcWestwolf.
• 2024-05-16 12:49:37.006493066 +0000 UTC m=+1743930.763628639: ☑️ agreed by lcwangchao.