config: support online reload for some config

[djshow832]

What problem does this PR solve?

Issue Number: close #94

Problem Summary:

• Some configurations can not be reloaded dynamically
• Some configurations may not be used. This will be a burden after GA.

What is changed and how it works:

• Remove selector-type because it's useless
• Make require-backend-tls able to be hot reloaded
• Remove server-version: the router reads the TiDB version and doesn't need a configured one
• Remove api.user, password, and enable-basic-auth: TiDB doesn't have these configs
• Make metrics able to be hot reloaded

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-10 03:49:00.674815712 +0000 UTC m=+3788938.261925856: ☑️ agreed by xhebox.
• 2023-11-10 06:18:25.997575909 +0000 UTC m=+3797903.584686054: ✖️🔁 reset by djshow832.
• 2023-11-10 07:08:11.069221163 +0000 UTC m=+3800888.656331308: ☑️ agreed by xhebox.

[djshow832]

/retest

[djshow832]

/retest