Issue #3162496: Support skipping field mappings where source is missing

Allow feeds to skip fields or field columns when a source is missing from the feed.

config/schema/feeds.schema.yml

@@ -215,6 +215,8 @@ feeds.processor.entity:
       type: boolean
     owner_id:
       type: integer
+    skip_missing_source:
+      type: boolean
 
 feeds.processor.entity:*:
   type: feeds.processor.entity