Issue #3214896: Add a plugin_selection_handler

config/schema/pluginreference.schema.yml

-# Schema for the configuration files of the Plugin Reference module.
+# Base schema for all plugin reference selection handler schemas.
+plugin_reference_selection:
+  type: mapping
+  label: 'Plugin reference selection handler settings'
+  mapping:
+    target_type:
+      type: string
+      label: 'Type of item to reference'
+
+# Schema for all plugin reference selection handlers that are not providing a
+# specific schema.
+plugin_reference_selection.*:
+  type: plugin_reference_selection
+
+# Schema for the plugin reference 'default' selection handler settings.
+plugin_reference_selection.default:
+  type: plugin_reference_selection
+  label: 'Default selection handler settings'
+  mapping:
+    sort:
+      type: mapping
+      label: 'Sort settings'
+      mapping:
+        key:
+          type: string
+          label: 'Sort by'
+        direction:
+          type: string
+          label: 'Sort direction'
+
+# Schema for all plugin reference 'default:*' selection handlers that are not
+# providing a specific schema.
+plugin_reference_selection.default:*:
+  type: plugin_reference_selection.default
+
+# Schema for all plugins that have a . in the name. e.g. field.widget and
+# field.formatter.
+plugin_reference_selection.default:*.*:
+  type: plugin_reference_selection.default
+
+field.field_settings.plugin_reference:
+  type: mapping
+  label: 'Plugin reference field settings'
+  mapping:
+    handler:
+      type: string
+      label: 'Reference method'
+    handler_settings:
+      type: plugin_reference_selection.[%parent.handler]
+      label: 'Plugin reference selection plugin settings'
+
+field.value.plugin_reference:
+  type: mapping
+  label: 'Default value'
+  mapping:
+    plugin_id:
+      type: string
+      label: 'Value'
+    configuration:
+      type: sequence
+      label: 'Configuration'
+      sequence:
+        type: string
 
 field.storage_settings.plugin_reference:
   type: mapping
@@ -12,6 +74,12 @@ field.widget.settings.plugin_reference_autocomplete:
   type: mapping
   label: 'Pluginreference autocomplete widget settings'
   mapping:
+    match_operator:
+      type: string
+      label: 'Autocomplete matching'
+    match_limit:
+      type: integer
+      label: 'Maximum number of autocomplete suggestions.'
     configuration_form:
       type: string
       label: 'How the plugin configuration form will be shown.'