*: move some common libraries to the client/pkg directory

[JmPotato]

What problem does this PR solve?

Issue Number: ref #8690.

What is changed and how does it work?

Move some common libraries to the `client/pkg` directory.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.76%. Comparing base (f91605a) to head (1ebee22). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8858 +/- ## ========================================== - Coverage 75.82% 75.76% -0.06% ========================================== Files 460 460 Lines 70637 70637 ========================================== - Hits 53557 53516 -41 - Misses 13702 13735 +33 - Partials 3378 3386 +8 ``` | [Flag](https://app.codecov.io/gh/tikv/pd/pull/8858/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/tikv/pd/pull/8858/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv) | `75.76% <ø> (-0.06%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=tikv#carryforward-flags-in-the-pull-request-comment) to find out more.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: okJiang, rleungx

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tikv/pd/blob/master/OWNERS)~~ [rleungx] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-11-26 09:24:48.692294787 +0000 UTC m=+542076.311949295: :ballot_box_with_check: agreed by rleungx.
• 2024-11-26 10:01:50.953990479 +0000 UTC m=+544298.573644995: :ballot_box_with_check: agreed by okJiang.