Add image convert (client-side)

[vvoland]

• needs: api/images/create: Allow creating image from passed OCI index moby/moby#48158
• needs: c8d: Multi-platform image list moby/moby#47526

- What I did
Add docker image convert command (working on the client side)

- How I did it

- How to verify it

convert --available

convert --platforms

convert --no-attestations

- Description for the changelog

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 104 lines in your changes missing coverage. Please review.

Project coverage is 61.16%. Comparing base (2f529a1) to head (b57527b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5249      +/-   ##
==========================================
- Coverage   61.46%   61.16%   -0.31%     
==========================================
  Files         298      299       +1     
  Lines       20820    20924     +104     
==========================================
  Hits        12798    12798              
- Misses       7110     7214     +104     
  Partials      912      912              

[cpuguy83]

I would also love to see a way to fetch the OCI index from the image get API.

[cpuguy83]

Sorry, intended for that to go into the moby PR, but works well here too :)