feat: Add min bound for page size, group header props, unit tests, new error codes, custom theming, thumbnail media support

[Ameera-Sherin]

Description:

• Unit Tests for Widgets: Implemented comprehensive unit tests to ensure the reliability and stability of widgets.
• Minimum Bound for Page Size: Added a minimum bound for page size to optimize user experience and performance.
• Mark as Read on Notification Card Click: Implemented the feature to mark notifications as read upon clicking on the notification card, enhancing user interaction and workflow.
• Group Header Parameters: Optimized the organization of header parameters for better readability and maintenance.
• Readme Update: Updated the README file to reflect the latest changes and provide comprehensive documentation for users and contributors.
• Dio Version Upgrade: Upgraded the Dio version to leverage the latest features, enhancements, and bug fixes.
• Handler function on Avatar click: Implemented a callback function that returns a Notification object upon clicking the Avatar handle.
• Add new error codes
• Add support for additional style and theming properties
• Rename functions, types, and arguments
• Support for flutter 3.0.0
• Custom theming
• Removed total elements
• Thumbnail media support