[1.0.0-beta2] Layout plugins support

Problem/Motivation

We are not supporting Layout plugins and dealing directly with SDC. This is compliant with the goal of Display Builder to be "design system" native.

However:

• some teams may want to use their already existing layout plugins from core, contrib or custom.
• if we do a migration mechanism from Layout Builder, we will need to support them

Proposed resolution

1. Add a LayoutSource plugin for slots. I would prefer to add it in display_builder and not ui_patterns, but that's open to discussion. This source will implement the upcoming SourcesBundlerInterface ( 📌 [1.0.0-alpha2] Remove ui_patterns_overrides Active )
2. Filter available layout plugins by removing the ones provided by ui_patterns, to avoid duplicates
3. Add the flattened source (each layout must be a draggable) in ComponentLibraryPanel