Issue #3479118: Add description to schema.

Closes #3479118

config/schema/external_entities.property_mapper.schema.yml

@@ -17,6 +17,9 @@ plugin.plugin_configuration.external_entities_property_mapper.constant:
       type: integer
       label: 'Debug level'
       nullable: true
+    description:
+      type: string
+      label: 'Description'
 
 plugin.plugin_configuration.external_entities_property_mapper.direct:
   type: config_object
@@ -42,6 +45,9 @@ plugin.plugin_configuration.external_entities_property_mapper.direct:
       type: integer
       label: 'Debug level'
       nullable: true
+    description:
+      type: string
+      label: 'Description'
 
 plugin.plugin_configuration.external_entities_property_mapper.simple:
   type: config_object
@@ -67,6 +73,9 @@ plugin.plugin_configuration.external_entities_property_mapper.simple:
       type: integer
       label: 'Debug level'
       nullable: true
+    description:
+      type: string
+      label: 'Description'
 
 plugin.plugin_configuration.external_entities_property_mapper.jsonpath:
   type: config_object
@@ -92,6 +101,9 @@ plugin.plugin_configuration.external_entities_property_mapper.jsonpath:
       type: integer
       label: 'Debug level'
       nullable: true
+    description:
+      type: string
+      label: 'Description'
 
 external_entities.external_entities_data_processor_settings:
   type: mapping