feat: add StacBackdropFilter widget

[manav-m]

Adds the StacBackdropFilter widget, which applies a filter to the background of its child widget. The backdrop filter is applied to everything behind the child widget that is within the same layer.

Examples
✅ Example: Basic blur filter

{
  "type": "clipRRect",
  "borderRadius": {
    "all": 16
  },
  "child": {
    "type": "sizedBox",
    "height": 200,
    "width": 300,
    "child": {
      "type": "stack",
      "fit": "expand",
      "children": [
        {
          "type": "container",
          "decoration": {
            "image": {
              "src": "https://images.pexels.com/photos/1323550/pexels-photo-1323550.jpeg",
              "fit": "cover"
            }
          }
        },
        {
          "type": "backdropFilter",
          "filter": {
            "type": "blur",
            "sigmaX": 15.0,
            "sigmaY": 15.0
          },
          "child": {
            "type": "container",
            "decoration": {
              "color": "#80FFFFFF"
            }
          }
        }
      ]
    }
  }
}

More examples can be found in the docs at docs/widgets/backdrop_filter.md

Related Issues
Closes #269

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Code refactor
• Build configuration change
• Documentation
• Chore

[CLAassistant]

All committers have signed the CLA.