Add Slike Player example page and enhance amp-slikeplayer functionality (#40381)

* Add Slike Player example page and enhance amp-slikeplayer functionality

- Introduced a new example HTML page for the Slike Player demonstrating its features and viewport behavior.
- Updated amp-slikeplayer.js to support automatic play/pause based on viewport visibility with configurable thresholds.
- Improved iframe source URL for better integration with the player service.

* Enhance Slike Player example and improve viewport handling

- Updated the Slike Player example HTML to improve structure and styling for better user experience.
- Enhanced amp-slikeplayer.js to include a method for handling auto play/pause based on viewport visibility.
- Fixed iframe source URLs in tests for better integration with the player service.

* Enhance amp-slikeplayer functionality and improve test coverage

- Added support for a poster image, creating a placeholder when provided.
- Updated iframe source handling to use `this.win.location.origin` for consistency.
- Implemented `isInteractive` method to indicate player interactivity.
- Enhanced tests to cover new features, including viewport handling and message posting for play/pause actions.
- Improved test structure for better readability and maintainability.

* Refactor test structure in amp-slikeplayer tests for clarity and consistency

Author: SlikeTech