manager, config: watch and apply the latest log configuration online

[djshow832]

What problem does this PR solve?

Issue Number: ref #94

Problem Summary:
Log configurations need to take effect dynamically.

What is changed and how it works:

• Watch the update of the log configuration and apply them to the LoggerManager.
• Manage the LoggerManager by Server and remove the logs in main().

TODO:
The LoggerManager relies on the ConfigManager to get the log path, but ConfigManager relies on LoggerManager to get the logger. Currently, LoggerManager logs to the path specified in the static config first and then switches to the path specified in the dynamic config. There are some ways to solve it:

• Forbid overwriting the log path
• Do not log in the ConfigManager before the logger is initialized
• There should be a global log path and a log path for each tenant. The global log path cannot be updated, but tenants can update theirs. Logging into different paths helps privilege control among tenants.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

$ echo '{"log-file": {"filename":"proxy.log"}}' | ./bin/tiproxyctl config log set
""
$ ./bin/tiproxyctl config log get
{"log-file":{"filename":"proxy.log"}}
$ cat proxy.log
[2022/09/23 10:27:05.069 +08:00] [INFO] [main.gin] [/api/admin/config/log] [status=200] [method=GET] [path=/api/admin/config/log] [query=] [ip=::1] [user-agent=Go-http-client/1.1] [latency=93.715µs]

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[xhebox]

Rest LGTM

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Rest LGTM