[2.0.4] Add PluginSettingsInterface::settingsSummary()

Problem/Motivation

SourceInterface is extending PluginSettingsInterface which is heavily inspired from Core's PluginSettingsInterface

We initially didn't add a method inspired from FormatterInterface::settingsSummary() to this interface, because we didn't have usage at the time.

The upcoming https://www.drupal.org/project/display_builder needs it.

Proposed resolution

1. Add method to our own PluginSettingsInterface
2. Implement this method in SourcePluginBase: if $form['value'] is the only settings (SourcePluginPropValueWidget?) and is scalar, print it.
3. Implement this method in slots sources:
   • BlockSource >> Selected block
   • ComponentSource >> Variant name
   • TokenSource >> $form['value'] (not the same as SourcePluginBase if we limit SourcePluginBase to SourcePluginPropValueWidget children)
   • WysiwygWidget >> The first characters (once tags are stripped) ?
   • FieldFormatterSourceDeriver >> ???
   • ViewFieldSource >> $form['ui_patterns_views_field']
   • ViewRowsSource >> $form['ui_patterns_views_field']

User interface changes

No

API changes

Nothing breaking.

Data model changes

No