cmd, server: add env to manage the service mode

[JmPotato]

What problem does this PR solve?

Issue Number: ref #5766.

What is changed and how does it work?

Add env `PD_SERVICE_MODE` to manage the service mode.

Check List

Tests

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

Release note

Support to bootstrap the PD service with environment variable `PD_SERVICE_MODE`.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• rleungx

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 345ebd6

[codecov]

Codecov Report

Merging #7018 (345ebd6) into master (da1b937) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7018      +/-   ##
==========================================
+ Coverage   74.16%   74.26%   +0.09%     
==========================================
  Files         433      433              
  Lines       46101    46101              
==========================================
+ Hits        34193    34235      +42     
+ Misses       8890     8858      -32     
+ Partials     3018     3008      -10     

Flag	Coverage Δ
unittests	74.26% <100.00%> (+0.09%)	⬆️

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