backendcluster: isolate topology fetch across clusters

[YangKeao]

What problem does this PR solve?

Issue Number: close #1149

Problem Summary:

What is changed and how it works:

1. Parallelize the GetTiDBTopology

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

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@2cdb1af). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1150   +/-   ##
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  Coverage        ?   67.42%           
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  Files           ?      149           
  Lines           ?    16275           
  Branches        ?        0           
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  Hits            ?    10973           
  Misses          ?     4564           
  Partials        ?      738           

Flag	Coverage Δ
unit	67.42% <100.00%> (?)

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