refactor(sigfetch): replace cjson with json

[MeteorsLiu]

A part of #377

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.32%. Comparing base (5b5bd4b) to head (4a4b4dd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #396   +/-   ##
=======================================
  Coverage   76.32%   76.32%           
=======================================
  Files          29       29           
  Lines        3016     3016           
=======================================
  Hits         2302     2302           
  Misses        673      673           
  Partials       41       41           

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[qiniu-x]

[Git-flow] Hi @MeteorsLiu, There are some suggestions for your information:

---

Rebase suggestions

• Following commits have duplicated messages

  fix: format json output

  fix: format json output

Which seems insignificant, recommend to use git rebase command to reorganize your PR.

For other git-flow instructions, recommend refer to these examples.

Details

If you have any questions about this comment, feel free to raise an issue here:

• https://github.com/qiniu/reviewbot