config, cmd: fix that the proxy configs are overwritten

[djshow832]

What problem does this PR solve?

Issue Number: close #690

Problem Summary:
When --advertise-addr is set, the whole proxy is overwritten by toml.Unmarshal(e.overlay, base), which results in all the fields are empty except for advertise-addr.

What is changed and how it works:

• Remove Sctx.Overlay and add Sctx.AdvertiseAddr
• Only set advertiseAddr explicitly in ConfigManager

Check List

Tests

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

1. Set the graceful-wait-before-shutdown in the config file
2. Start TiProxy with --advertise-addr
3. Start TiProxy and check the log, the config shows graceful-wait-before-shutdown correctly

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.

- Fix that the `proxy` configurations are overwritten when the command line `--advertise-addr` is set.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@4471d26). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/server/server.go	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #691   +/-   ##
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  Coverage        ?   70.35%           
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  Files           ?      103           
  Lines           ?     9891           
  Branches        ?        0           
=======================================
  Hits            ?     6959           
  Misses          ?     2443           
  Partials        ?      489           

Flag	Coverage Δ
unit	70.35% <66.66%> (?)

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[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:

• 2024-09-27 01:54:21.971167619 +0000 UTC m=+1790131.711591823: ☑️ agreed by xhebox.