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