Support nesting the entire document under a different key

[graytaylor0]

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.

[dlvenable]

I like the idea of using / for the entire event.

[dlvenable]

I think if we use /, we can use existing processors such as rename_keys.

[oeyh]

In JsonPointer specification, empty string ("") refers to the whole document and / refers to field with empty string as key.

I followed that way and made some changes in #4033 so that JacksonEvent#get("", Object.class) will get you the entire document. Currently "" is not allowed to use in JacksonEvent#put, but we can allow that to put objects to root of the document.

[dlvenable]

Thanks @oeyh for clarifying that! So then I agree that "" would be the right way to express this.