br: configure the httpclient for external storage (#46040)

[ti-chi-bot]

This is an automated cherry-pick of #46040

What problem does this PR solve?

Issue Number: close #46011

Problem Summary:
br use the default http client to connect the external storage, which doesn't have enough reuse-connection-pool for each host(the default value is 2). This leads to slow batch files processing and TCP connections exceeding the system configuration limit.

What is changed and how it works?

set the MaxIdleConnsPerHost as the same as MaxIdleConns.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

before configuration

after configuration

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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

None

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@617a8ff). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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##             release-6.5     #46140   +/-   ##
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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3pointer, Leavrth

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• OWNERS [3pointer,Leavrth]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-17 04:04:02.778886282 +0000 UTC m=+776607.327902254: ☑️ agreed by Leavrth.
• 2023-08-17 05:08:30.792241624 +0000 UTC m=+780475.341257596: ☑️ agreed by 3pointer.