Make cli plugin log purge recognize steps by name

[smainz]

Give a little love to woodpecker-cli pipeline log purge:

• Make it recognize steps by name
• Improve error messages in case of invalid/missing args
• Add step name to message if only logs of a single stepare purged

closes #3922

[qwerty287]

This closes #3922 right?

[smainz]

yes, forgot to add this.

[woodpecker-bot]

Deployment of preview was successful: https://woodpecker-ci-woodpecker-pr-3953.surge.sh

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 26.87%. Comparing base (fed8093) to head (f167769).
Report is 2 commits behind head on main.

Files	Patch %	Lines
cli/log/log_purge.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3953      +/-   ##
==========================================
+ Coverage   26.83%   26.87%   +0.04%     
==========================================
  Files         394      394              
  Lines       27510    27517       +7     
==========================================
+ Hits         7381     7395      +14     
+ Misses      19428    19422       -6     
+ Partials      701      700       -1     

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