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

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None

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