feat: Add extraLinesData support to CandlestickChart

[owenmoon1]

Feature: Add extraLinesData support to CandlestickChart

Adds extraLinesData support to the CandlestickChart, implementing the same functionality available in LineChart.

Changes:

• ✅ Added extraLinesData parameter to CandlestickChartData
• ✅ Updated painter to draw horizontal and vertical extra lines
• ✅ Added equality tests for the new functionality
• ✅ Updated example app with median price line demonstration
• ✅ Updated CHANGELOG.md

Use Case:

This feature enables traders and developers to draw reference lines on candlestick charts, such as:

• Support/Resistance levels
• Moving averages
• Median prices
• Any custom horizontal or vertical reference lines

Example:

See the updated candlestick_chart_sample1.dart for a working example showing a yellow median price line across Bitcoin prices.

Testing:

• ✅ All 564 tests passing
• ✅ No linter errors
• ✅ Code properly formatted

[imaNNeo]

Thanks, Owen, it already looks good to me.
Can you please update our candlestick_chart.md file to add the new property there?

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.47%. Comparing base (32e75f5) to head (808c5a8).

Files with missing lines	Patch %	Lines
...t/candlestick_chart/candlestick_chart_painter.dart	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2016      +/-   ##
==========================================
- Coverage   92.49%   92.47%   -0.02%     
==========================================
  Files          50       50              
  Lines        3731     3735       +4     
==========================================
+ Hits         3451     3454       +3     
- Misses        280      281       +1     

Flag	Coverage Δ
flutter	92.47% <75.00%> (-0.02%)	⬇️

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[imaNNeo]

Can you please add the unit tests for the new changes?