etcdctl: refactor json printing to support custom writers

[kstrifonoff]

A necessary "pre-change" for #19469.

The jsonPrinter struct now includes an io.Writer field, enabling unit testing of this module.

[k8s-ci-robot]

Hi @kstrifonoff. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[kstrifonoff]

/cc @ahrtr

[ahrtr]

/ok-to-test

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 69.25%. Comparing base (db4dfbd) to head (e6be3f3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdctl/ctlv3/command/printer_json.go	0.00%	8 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdctl/ctlv3/command/printer_json.go	0.00% <0.00%> (ø)

... and 148 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #20177      +/-   ##
==========================================
+ Coverage   62.67%   69.25%   +6.57%     
==========================================
  Files         401      413      +12     
  Lines       33292    34367    +1075     
==========================================
+ Hits        20867    23802    +2935     
+ Misses      10867     9173    -1694     
+ Partials     1558     1392     -166     

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[ahrtr]

LGTM & thx

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, fuweid, kstrifonoff

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