config, router: add health check switch

[djshow832]

What problem does this PR solve?

Issue Number: close #205

Problem Summary:
The serverless tier doesn't need health check, because the standby tidb might be unhealthy.

What is changed and how it works:

• Add a member Enable to config.HealthCheck.
• Move router.HealthCheckConfig to config.HealthCheck and rename some members.
• Add a parameter *config.HealthCheck to NewScoreBasedRouter().

TODO:

• Add config.HealthCheck to the configuration file. Currently, it's still using the default value.
• The config.HealthCheck parameter of ScoreBasedRouter and PDFetcher is duplicated and should be refined.
• Unit tests

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 (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

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None