feat: add indicatorSpacing, indicatorLeadingWidget, and indicatorTrailingWidget to Carousel

[usmanvrtx]

Description

This PR adds three new properties to the Carousel widget to enhance indicator customization:

• indicatorLeadingWidget: A custom widget displayed before the indicator row or column.
• indicatorTrailingWidget: A custom widget displayed after the indicator row or column.

These additions allow developers to place custom widgets alongside the carousel indicators for richer UI interactions.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added indicatorLeadingWidget, and indicatorTrailingWidget fields to the Carousel schema definition (ensemble_schema.json)
• Implemented the corresponding controller fields and state widget builders in carousel.dart
• Updated the indicator rendering logic to support leading/trailing widgets with configurable spacing

How to Test

1. Add indicatorLeadingWidget and/or indicatorTrailingWidget to a Carousel widget definition
2. Verify the widgets render correctly in both horizontal and vertical orientations

Screenshots / Videos

N/A

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors