opts: implement PlatformSlice option

[thaJeztah]

• follow-up to image rm: add --platform option #6109

I had these changes in a local branch, but it wasn't finished yet; TODO;

• add tests
• decide whether we want this in the opts package, or better to have it in a separate package (due to extra dependencies involved)
• do we need a similar options for single platforms?
• should this package include utilities for tab-completion?
• should the String() format be symmetrical with the input value (no space between values?)

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.80645% with 23 lines in your changes missing coverage. Please review.

Project coverage is 54.79%. Comparing base (28f19a9) to head (7c8160f).
Report is 4 commits behind head on master.

❌ Your patch status has failed because the patch coverage (25.80%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6166      +/-   ##
==========================================
- Coverage   54.82%   54.79%   -0.04%     
==========================================
  Files         364      365       +1     
  Lines       30467    30483      +16     
==========================================
- Hits        16703    16702       -1     
- Misses      12789    12807      +18     
+ Partials      975      974       -1     

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