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]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.25%. Comparing base (36972d6) to head (d2f4172).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdctl/ctlv3/command/global.go	0.00%	67 Missing ⚠️
etcdctl/ctlv3/command/fakeclient/client.go	0.00%	31 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	0.00% <0.00%> (ø)
etcdctl/ctlv3/ctl.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/fakeclient/client.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/global.go	0.00% <0.00%> (ø)

... and 24 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21230      +/-   ##
==========================================
- Coverage   68.45%   68.25%   -0.20%     
==========================================
  Files         429      430       +1     
  Lines       35288    35341      +53     
==========================================
- Hits        24156    24122      -34     
- Misses       9746     9817      +71     
- Partials     1386     1402      +16     

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[hwdef]

cc @siyuanfoundation