Support nesting the entire document under a different key

Is your feature request related to a problem? Please describe.
As a user of Data Prepper, my Events look like this

{
 "key-one": "value-one",
 "key-two": "value-two"
}

and I would like them to look like this

{
  "new-key": {
       "key-one": "value-one",
       "key-two": "value-two"
   }
}

Describe the solution you'd like
A way to pass the root (or entire existing document) of an Event to json pointer expression. For example, this could be with /, allowing add_entries to add keys at the highest level with the existing document

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

The issue names the add_entries processor and JSON Pointer expressions, but no files or tests. Start by tracing how add_entries resolves its target path and how the existing document is represented; confirm the behavior with the repository's current add_entries tests or examples. Done means an event can be nested under a new key while preserving its existing fields.