[Feature Request] Add FlDotImagePainter to support custom image dot markers

[InsoooooooooJANG]

Is your feature request related to a problem? Please describe.

Currently, fl_chart only supports basic geometric shapes (circle, square, cross) for dot markers on line charts through FlDotCirclePainter, FlDotSquarePainter, and FlDotCrossPainter.

However, many use cases require custom visual markers such as:

• User profile pictures on activity charts
• Product icons on sales trend charts
• Custom branded icons for different data series
• Weather icons on temperature charts

There's no built-in way to use custom images as dot markers, forcing developers to either:

1. Use the limited geometric shapes
2. Implement complex custom painters from scratch
3. Use workarounds that don't integrate well with the existing dot system

Describe the solution you'd like

I propose adding a FlDotImagePainter class that extends FlDotPainter to support rendering custom images as dot markers.

Proposed API:

// Load image asynchronously
final image = await FlDotImagePainter.loadImageFromAsset('assets/icon.png');
final painter = FlDotImagePainter(image: image, size: 20.0);

// Use in chart
LineChart(
  LineChartData(
    lineBarsData: [
      LineChartBarData(
        dotData: FlDotData(
          show: true,
          getDotPainter: (spot, percent, barData, index) => painter,
        ),
      ),
    ],
  ),
)