repro: parallel execution of stages

[sjawhar]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

TODO

• Tests
• Add --jobs argument to CLI

[codecov]

Codecov Report

❌ Patch coverage is 94.69027% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.96%. Comparing base (2431ec6) to head (830f807).
⚠️ Report is 163 commits behind head on main.

Files with missing lines	Patch %	Lines
dvc/repo/reproduce.py	90.16%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10922      +/-   ##
==========================================
- Coverage   90.68%   84.96%   -5.72%     
==========================================
  Files         504      502       -2     
  Lines       39795    40892    +1097     
  Branches     3141     3232      +91     
==========================================
- Hits        36087    34744    -1343     
- Misses       3042     5434    +2392     
- Partials      666      714      +48     

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