[11.2.0-and-up] Add support for SDC variants

Overview

In ✨ Introduce component variants to SDC Active we added SDC variants to SDC.
We need to provide support for it on XB.

Proposed resolution

• Allow to select the variant when an SDC schema supports them.
• Expand \Drupal\Tests\experience_builder\Kernel\Plugin\ExperienceBuilder\ComponentSource\SingleDirectoryComponentTest with tests

User interface changes

Editor will be able to select a variant.