support 3rd party settings in field formatters

Closes #3487548

modules/ui_patterns_blocks/tests/fixtures/config/block.block.test_block.yml

@@ -27,7 +27,7 @@ settings:
         value: ''
     props:
       attributes:
-        source_id: class_attribute
+        source_id: attributes
         source:
           value: ''
       string:
@@ -93,15 +93,15 @@ settings:
         source:
           value: ''
       attributes_implicit:
-        source_id: class_attribute
+        source_id: attributes
         source:
           value: ''
       attributes_ui_patterns:
-        source_id: class_attribute
+        source_id: attributes
         source:
           value: ''
       attributes_class:
-        source_id: class_attribute
+        source_id: attributes
         source:
           value: ''
     slots: