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

**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:**
```dart
// 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,
),
),
],
),
)

Start by reading the existing FlDotPainter, FlDotCirclePainter, FlDotSquarePainter, and FlDotCrossPainter entry points, then review how LineChartBarData and FlDotData provide dot painters. Define the image-loading and rendering behavior described in the proposal, and consider the feature done when custom asset images can be used as line-chart dot markers through the proposed API.

Written by the indexing model from the issue text.