[2.0.0-beta1] Make Form Builder and/or Source plugins forms extendable

Add a way of extending Form Builder and/or Source plugins forms.

It will be needed both :

• https://www.drupal.org/project/ui_patterns_settings → 3.x
• As a follow up of 🐛 [2.0.0-alpha2] Formatters: Remove Twig-based slot rewrite feature Fixed