Feature: #84159 - Extract admin panel to own extension

See Issue #84159

Description

The admin panel has been extracted to a standalone extension. All admin panel specific code will be moved to the extension removing cross-dependencies and enabling better scoping.

Impact

The admin panel can be completely uninstalled by deactivating the extension. To use the admin panel functionality the extension has to be activated. Classes have been moved to the new extension and a class alias map for migration of legacy code has been provided.