etcdctl: refactor global flags management and simplify command initialization

[hwdef]

This PR is a part of: https://github.com/etcd-io/etcd/pull/20270

It prepares for subsequent unit testing.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 89 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 69.42%. Comparing base (7b1f0f2) to head (c85f68d). :warning: Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
etcdctl/ctlv3/command/global.go	0.00%	78 Missing :warning:
etcdctl/ctlv3/command/make_mirror_command.go	0.00%	5 Missing :warning:
etcdctl/ctlv3/command/ep_command.go	0.00%	4 Missing :warning:
etcdctl/ctlv3/command/check.go	0.00%	1 Missing :warning:
etcdctl/ctlv3/ctl.go	0.00%	1 Missing :warning:

Additional details and impacted files

Files with missing lines	Coverage Δ
etcdctl/ctlv3/command/check.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/ctl.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/ep_command.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/make_mirror_command.go	0.00% <0.00%> (ø)
etcdctl/ctlv3/command/global.go	0.00% <0.00%> (ø)

... and 22 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21010      +/-   ##
==========================================
+ Coverage   69.21%   69.42%   +0.21%     
==========================================
  Files         422      422              
  Lines       34841    34794      -47     
==========================================
+ Hits        24114    24156      +42     
+ Misses       9328     9245      -83     
+ Partials     1399     1393       -6     

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[hwdef]

/retest

[hwdef]

/hold