add custom pip cmd (uv) support

[krisl]

adds support to use any pip command, like maybe "uv pip"

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.09%. Comparing base (ad3a610) to head (874b292).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/rosdep2/platforms/pip.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1010      +/-   ##
==========================================
+ Coverage   71.76%   72.09%   +0.33%     
==========================================
  Files          44       44              
  Lines        3488     3497       +9     
  Branches      686      689       +3     
==========================================
+ Hits         2503     2521      +18     
+ Misses        808      797      -11     
- Partials      177      179       +2     

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