New processor: copy_fields (elastic#11303)

A new processor is introduced as part of support for keeping orignal messages. Options and naming follows the convention of other processors.

### `copy_fields`

This processor copies one field to another. Example configuration is below:

```yaml
processors:
- copy_fields:
    fields:
      - from: message
        to: event.original
    fail_on_error: false
    ignore_missing: true
```

journalbeat.reference.yml

@@ -269,6 +269,16 @@ setup.template.settings:
 #    max_depth: 1
 #    target: ""
 #    overwrite_keys: false
+#
+# The following example copies the value of message to message_copied
+#
+#processors:
+#- copy_fields:
+#    fields:
+#        - from: message
+#          to: message_copied
+#    fail_on_error: true
+#    ignore_missing: false
 
 #============================= Elastic Cloud ==================================