api: add /health/unhealthy endpoint to mark the instance as unhealthy

[YangKeao]

What problem does this PR solve?

Issue Number: close #1140

Problem Summary:

For the LB from cloud provider, once the Pod enters terminating status, the connection will persist at most 15min (for aliyun) and 1h (for AWS). However, for unhealthy, the connection can be persisted for a very long time: 100h for AWS and not limited for Aliyun (I've only tested for 1h, and I'll try 24h tests later).

What is changed and how it works:

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]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign djshow832 for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@2c402e5). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1141   +/-   ##
=======================================
  Coverage        ?   67.44%           
=======================================
  Files           ?      149           
  Lines           ?    16128           
  Branches        ?        0           
=======================================
  Hits            ?    10878           
  Misses          ?     4514           
  Partials        ?      736           

Flag	Coverage Δ
unit	67.44% <100.00%> (?)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.