etcdctl: refactor global flags management and simplify command initialization

[hwdef]

This PR is a part of: #20270

It prepares for subsequent unit testing.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 52 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.39%. Comparing base (36972d6) to head (fc8bef4).
⚠️ Report is 215 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdctl/ctlv3/command/global.go	0.00%	46 Missing ⚠️
etcdctl/ctlv3/command/util.go	0.00%	2 Missing ⚠️
etcdctl/ctlv3/ctl.go	0.00%	2 Missing ⚠️
etcdctl/ctlv3/command/diagnosis_command.go	0.00%	1 Missing ⚠️
etcdctl/ctlv3/command/ep_command.go	0.00%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdctl/ctlv3/command/diagnosis_command.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/ep_command.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/util.go	7.29% <0.00%> (+7.29%)	⬆️
etcdctl/ctlv3/ctl.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/global.go	0.00% <0.00%> (ø)

... and 53 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21230      +/-   ##
==========================================
- Coverage   68.45%   68.39%   -0.06%     
==========================================
  Files         429      428       -1     
  Lines       35288    35292       +4     
==========================================
- Hits        24156    24139      -17     
- Misses       9746     9754       +8     
- Partials     1386     1399      +13     

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[hwdef]

cc @siyuanfoundation

[hwdef]

@siyuanfoundation Thanks for your replay. I have deleted the unnecessary code.

[siyuanfoundation]

Overall I think the PR looks good. And not using GlobalFlags is beneficial for future changes of adding a cmd based on fake client.
@hwdef Can you squash the commits?

[hwdef]

Thank you for your reply. I have already squashed the commits.