feat: matrix map rows

[sjawhar]

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• 📖 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. 🙏

Adds support for maps in matrix rows, e.g. the foo in

# dvc.yaml
stages:
  foobar:
    matrix:
      foo:
        goo: 1
        baz: 2
      bar: [1, 2, 3]
    cmd: python foobar.py --foo ${item.foo} --bar ${item.bar}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (49c3b98).
⚠️ Report is 175 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10943      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41071    +1276     
  Branches     3141     3249     +108     
==========================================
+ Hits        36087    37370    +1283     
- Misses       3042     3063      +21     
+ Partials      666      638      -28     

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