feat: add gradient support for BarChartRodStackItem

[kamilJ96]

Resolves #919

This PR adds a gradient property on BarChartRodStackItem, which displays a stacked chart rod with a gradient if provided.

Saw there was an attempt at this already, but no activity on that for a few weeks now and I'm needing this feature and impatient 😛

Unfortunately dart doesn't support both optional positional arguments and named arguments, so the constructor signature is not ideal, however this avoids a breaking change as opposed to updating color to a named argument. If there's a better way, or we make that breaking change, let me know.

I've also updated a sample bar chart to show the effect.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.51%. Comparing base (06835da) to head (f865292).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1979   +/-   ##
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  Coverage   92.51%   92.51%           
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  Files          50       50           
  Lines        3712     3714    +2     
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+ Hits         3434     3436    +2     
  Misses        278      278           

Flag	Coverage Δ
flutter	92.51% <100.00%> (+<0.01%)	⬆️

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[imaNNeo]

Simple and neat!
Thank you very much for your nice PR