Add possibility for plugins to add a HTML file

INFO

This document represents an architecture decision record (ADR) and has been mirrored from the ADR section in our Shopware 6 repository. You can find the original version here

Context

The new ExtensionAPI is based on a iFrame communication architecture. The old App system for the admin relies on the XML file. And the normal plugin architecture in the admin is based on component overriding. The ideal way for developing admin extensions will be the ExtensionAPI.

Decision

To provide a smooth transition for plugin developer to the new ExtensionAPI which will be introduced soon we need to make sure that plugin can also behave like Apps in the administration. To fulfill this we need to provide a solution to show their own iFrame views. This is now directly possible when the plugin developer adds a index.html file to the plugin in the administration folder.

This file will automatically be used by webpack and can be used like a normal web application.