Symmetric config translation

Problem/Motivation

UI Patterns is currently building a symmetric translation system for content storage only: ✨ [trans] Make SourceValueItem field type translatable with synchronized and asynchron translations Active where:

• translations are flat list of translatable sources with their tree node IDs
• we rebuild the translated tree by overriding tree nodes values with translated values

On Display Builder side, we will leverage this feature and we need to do the same for our config entities: Page Layout, Views & Entity View Display.

Proposed resolution

1. Make source property translatable
2. Add a way of retrieving all nodes with translatable source plugins
3. In ConfigTranslationAddForm and ConfigTranslationEditForm, expose those translatable source with the UI Patterns Form Builder (the final goal is to translate from Display Builder UI, but this will be a post-beta 1 target)
4. Make it store a flat list of nodes and build the full translated field when retrieving the value