utils: let http client can reuse connections

[HuSharp]

What problem does this PR solve?

Issue Number: Close #xxx

What is changed and how does it work?

cherry pick #6917

Check List

Tests

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

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• nolouch

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[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
pkg/utils/etcdutil/etcdutil.go	100.00%

📢 Thoughts on this report? Let us know!.

[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 0554e20

[purelind]

/test build

[ti-chi-bot]

@purelind: No presubmit jobs available for tikv/pd@release-7.1-20230906-v7.1.1

In response to this:

/test build

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[purelind]

/test build