http_server: add http handler for failpoint (#1722)

[ti-srebot]

cherry-pick #1722 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In ticdc repo:
git pr https://github.com/pingcap/ticdc/pull/1732

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/ticdc.git pr/1732:release-4.0-e4e96ed4204a

---

What problem does this PR solve?

• Lack of a convenient way to control failpoints at runtime.

What is changed and how it works?

• Added an http API endpoint as /fail/

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch

Release note

• Aadd http handler for failpoint

[ti-srebot]

/run-all-tests

[codecov-commenter]

Codecov Report

Merging #1732 (d0a5483) into release-4.0 (753073a) will increase coverage by 0.0559%.
The diff coverage is 100.0000%.

@@                 Coverage Diff                 @@
##           release-4.0      #1732        +/-   ##
===================================================
+ Coverage      53.7661%   53.8220%   +0.0559%     
===================================================
  Files              153        153                
  Lines            15958      15960         +2     
===================================================
+ Hits              8580       8590        +10     
+ Misses            6475       6468         -7     
+ Partials           903        902         -1     

[zier-one]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• leoppro

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[liuzix]

/lgtm

[liuzix]

/merge

[ti-chi-bot]

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

Commit hash: d0a5483

[amyangfei]

/run-all-tests

[amyangfei]

/run-leak-tests