Add rounded iframe sample

[ttw225]

This PR adds a new sample sample.rounded-iframe, which demonstrates how to use an iframe with rounded corners in a Chrome Extension.

The inspiration for this template stems from discussions across various platforms, including Stack Overflow and the Chrome Extensions Samples repository, where developers have explored methods to implement rounded corners in extensions. Notably, the Google Keep Chrome Extension has been recognized for its effective use of rounded corners. Inspired by its advanced techniques, this template simplifies and adapts those methods to assist developers in creating more visually appealing extension interfaces.