Add SingleOrVec::iter

[fuzzypixelz]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 70.83%. Comparing base (16ba77c) to head (a92068e).
Report is 16 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
commons/zenoh-collections/src/single_or_vec.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1848      +/-   ##
==========================================
- Coverage   70.92%   70.83%   -0.09%     
==========================================
  Files         360      360              
  Lines       65175    65194      +19     
==========================================
- Hits        46225    46183      -42     
- Misses      18950    19011      +61     

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