tiproxy: fix http tls when only tls cluster is enabled

[xhebox]

What problem does this PR solve?

Before, tiproxy will use TLS certs from TiDB to serve both SQL and HTTP ports. When only tlsCluster is enabled, there will be errors(others will try to use HTTPS to access tiproxy).

Since now we have dedicated server-http-tls options, we can now specify certs for tiproxy when only tlsCluster is enabled.

For backward compatibility, it is also enabled on only tlsTiDB, which is wrong. But it is kept for now.

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.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 61.29%. Comparing base (521a829) to head (0f21834).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5555      +/-   ##
==========================================
- Coverage   61.32%   61.29%   -0.03%     
==========================================
  Files         235      235              
  Lines       30080    30083       +3     
==========================================
- Hits        18446    18440       -6     
- Misses       9792     9798       +6     
- Partials     1842     1845       +3     

Flag	Coverage Δ
unittest	61.29% <0.00%> (-0.03%)	⬇️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csuzhangxc

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-26 01:53:04.013343854 +0000 UTC m=+840472.760966967: ☑️ agreed by csuzhangxc.