Padding and Indexing Functionality

[natty-pluz]

The flutter_grouped_scroll_view package lacks two key functionalities for customizing list appearance and behavior:

1. Padding: Unlike ListView and GridView, it doesn't offer padding options, making it difficult to add space between list content and the viewport edges.

2. Indexing: There's no way to access individual list item indexes within the builder function. This limits the ability to customize list tiles based on their position (e.g., changing background color based on even/odd indices).

Proposed Solution:

This issue proposes adding these functionalities to flutter_grouped_scroll_view:

1. Padding: Introduce a new property that allows specifying padding around the list content.
   e.g., padding: EdgeInsets.all(8)

2. Indexing: Provide a way to access the index of each list item within the item builder function.
   e.g., itemBuilder: (context, item, index)