[Feature Request]: JSON Input - when processing a field, it can only be a String

### What would you like to happen?

You can configure a JSON Input transform to extract data from a field from the previous transform, especially if you need to decode a complex-structured payload or a REST response.

However, this field can only be a String: it you try to pass on a JSON object (as you're probably tempted to do, when linking more JSON Input transforms together), the transform fails for a ClassCastException.

### Issue Priority

Priority: 3

### Issue Component

Component: Transforms

Start by locating the JSON Input transform and the field-processing code that currently raises the ClassCastException for non-String values. Reproduce the issue with a JSON object passed from a previous transform, then verify that chained JSON Input transforms can process the object successfully.

Written by the indexing model from the issue text.