RFC: Certificate-Based Authentication

[djshow832]

What problem does this PR solve?

Issue Number: ref #464

Problem Summary:
This proposes a design of supporting certificate-based authentication on TiProxy.

What is changed and how it works:
Add a RFC.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@1c32cf8). Click here to learn what that means.

Additional details and impacted files

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  Coverage        ?   64.43%           
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  Files           ?       61           
  Lines           ?     5809           
  Branches        ?        0           
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  Hits            ?     3743           
  Misses          ?     1779           
  Partials        ?      287           

Flag	Coverage Δ
unit	64.43% <ø> (?)

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