Supporting iterate_on for write_json

Is your feature request related to a problem? Please describe.
When using write_json on a key with an array value, I would like to be able to convert each element in the array to its json string representation. Currently if an array is passed to write_json it will generate the json string representation of the array.

For example currently

"key1": [
{"a":"b", "c":"d"}
]

with the processor

- write_json:
    - source: key1
      target: some_key

outputs

"some_key": "[{\"a\":\"b\",\"c\":\"d\"}]"

But with iterate_on it would look something like this

"some_key": [
"{\"a\":\"b\",\"c\":\"d\"}",
]

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Additional context
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Start by locating the write_json processor implementation and its tests, then compare them with the existing iterate_on behavior in other processors. Confirm how array inputs are currently serialized and define tests showing that iterate_on produces one JSON string per array element while preserving current behavior without it.