parse_json: add ability to only parse top level fields

[cameronattard]

Is your feature request related to a problem? Please describe. Nested json parsing can introduce challenges with OpenSearch field limits and type conflicts between object and concrete field types. It would be helpful to be able to only parse the top level of JSON fields, leaving subfields as strings.

Describe the solution you'd like Add an option to only parse the top level of a JSON-encoded string field.

Describe alternatives you've considered (Optional) Perform JSON parsing elsewhere, e.g. fluent-bit.

Additional context N/A