Add kn secret command

[dsimansk]

Description

Changes

• 🎁 kn secret cmd group

Reference

Fixes #287

Release Note

Add `kn secret` commands group for managing secrets

PTAL do you recall this feature? :)
/cc @rhuss

[codecov]

Codecov Report

Patch coverage: 62.57% and project coverage change: -0.18 ⚠️

Comparison is base (7586217) 79.87% compared to head (59c6fb7) 79.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1791      +/-   ##
==========================================
- Coverage   79.87%   79.70%   -0.18%     
==========================================
  Files         174      179       +5     
  Lines       13593    13850     +257     
==========================================
+ Hits        10858    11039     +181     
- Misses       1992     2050      +58     
- Partials      743      761      +18     

Impacted Files	Coverage Δ
pkg/kn/commands/types.go	82.60% <23.07%> (-5.23%)	⬇️
pkg/kn/commands/secret/create.go	53.12% <53.12%> (ø)
pkg/kn/commands/secret/list.go	70.96% <70.96%> (ø)
pkg/kn/commands/secret/delete.go	71.42% <71.42%> (ø)
pkg/kn/commands/secret/secret.go	100.00% <100.00%> (ø)
pkg/kn/root/root.go	90.00% <100.00%> (+0.08%)	⬆️

... and 4 files with indirect coverage changes

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[vyasgun]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, vyasgun

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