tiproxy: support set require-backend-tls and use separate config for tiproxy HTTP port

[xhebox]

What problem does this PR solve?

Latest tiproxy does not require this workaround anymore. require-backend-tls is false by default. Support resetting the config by user.

Also, support server-http-tls config, which will be used by newer tiproxy. This is set to skip-ca=true so that tiproxyctl or NLB wont need to provide any cert even if server-tls.skip-ca=false. ref pingcap/tiproxy#403

Old configs are kept as is by Get() == nil, no changes should be made.

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csuzhangxc

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[codecov-commenter]

Codecov Report

Merging #5389 (25f807d) into master (d8426ec) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5389      +/-   ##
==========================================
- Coverage   61.61%   61.59%   -0.02%     
==========================================
  Files         228      228              
  Lines       28895    28901       +6     
==========================================
- Hits        17804    17803       -1     
- Misses       9347     9354       +7     
  Partials     1744     1744              

Flag	Coverage Δ
unittest	61.59% <0.00%> (-0.02%)	⬇️

[xhebox]

/test all

[ti-chi-bot]

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/test all

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-13 06:40:49.954613662 +0000 UTC m=+4058447.541723804: ☑️ agreed by csuzhangxc.
• 2023-11-13 15:08:56.799041744 +0000 UTC m=+4088934.386151874: ✖️🔁 reset by ti-chi-bot[bot].

[ti-chi-bot]

New changes are detected. LGTM label has been removed.

[ti-chi-bot]

@xhebox: Your PR was out of date, I have automatically updated it for you.

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