mcs: add config api for tso service

[rleungx]

What problem does this PR solve?

Issue Number: Ref #5839.

What is changed and how does it work?

This PR adds a config API so that TiDB can use query to get the config of the TSO server.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• lhy1024

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[codecov]

Codecov Report

Merging #7858 (533e48b) into master (e199866) will decrease coverage by 0.07%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head 533e48b differs from pull request most recent head 590a576. Consider uploading reports for the commit 590a576 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7858      +/-   ##
==========================================
- Coverage   73.52%   73.46%   -0.07%     
==========================================
  Files         432      432              
  Lines       47937    47958      +21     
==========================================
- Hits        35248    35230      -18     
- Misses       9650     9688      +38     
- Partials     3039     3040       +1     

Flag	Coverage Δ
unittests	73.46% <100.00%> (-0.07%)	⬇️

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[JmPotato]

Is it possible to panic here?

[rleungx]

seem we need to use Get?

[rleungx]

But I think it barely happens.

[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: 76b3279

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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