Feature Request: Add gradient support to FlDotPainter for custom dot rendering

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Hi Dear Iman,

First of all, thank you for the great work on `fl_chart` — it's one of the best charting solutions for Flutter and we've really enjoyed using it.

While customizing line chart dots using `FlDotData` and `getDotPainter`, I noticed that there's currently no built-in support for applying gradient styles to the dots. Right now, we can only use solid colors in `FlDotCirclePainter` or other `FlDotPainters`, which limits the visual styling options — especially when trying to highlight important data points (like the last point) with gradients.

## Feature Request:

I’ve already implemented a custom `GradientDotPainter` in my project using a custom painter, and it works well. I’d be happy to contribute this as a Pull Request if you're open to the idea.

Adding this feature would unlock more flexibility and polish in custom charts and improve visual storytelling.

Thanks again for your awesome work and for considering this request.

Best regards.

Start by reading the FlDotData, getDotPainter, FlDotPainter, and FlDotCirclePainter APIs to understand how custom dot rendering currently works. Compare the requested behavior with the reported custom GradientDotPainter approach, and consider the feature complete when dot painters can apply gradients while preserving existing solid-color behavior and documented usage.

Written by the indexing model from the issue text.