api: support get json format for config by setting Accept header

[djshow832]

What problem does this PR solve?

Issue Number: close #483

Problem Summary:
/api/admin/config doesn't respect Accept field in the request header, while some applications may expect a json format.

What is changed and how it works:
If Accept is json or format is json, return a json format.

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.

- Support get JSON format config for /api/admin/config by setting Accept='application/json' in the request header

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@5d2dfc2). Click here to learn what that means.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #484   +/-   ##
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  Coverage        ?   64.18%           
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  Files           ?       61           
  Lines           ?     5786           
  Branches        ?        0           
=======================================
  Hits            ?     3714           
  Misses          ?     1783           
  Partials        ?      289           

Flag	Coverage Δ
unit	64.18% <100.00%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-15 14:37:57.120019345 +0000 UTC m=+1041904.142265726: ☑️ agreed by xhebox.