Feature request: Allow json_transforms prior to json_path or image_url_path usage.

I have experimented with initializing a PyPortal object using `json_transform=(myTransform),`. Based on debug print statements, the transform function I wrote seems to be called after the `.fetch()` method has completed (or at least at the very end of the `.fetch()` method). I want to be able to edit (transform) the JSON/dict immediately upon it's creation -- before the `image_json_path` (or the `json_path`) is put to use.

For example, I'm trying to adapt the https://learn.adafruit.com/cleveland-museum-of-art-pyportal-frame project to the Art Institute of Chicago's Public API. The Cleveland project uses `image_json_path` to locate a .JPG file to display. For the Art Institute of Chicago, the .JPG location can only be found by concatenating the values from **_two separate_** key:value pairs and a couple of string constants.

I.e., my json_transform would

`json_out["joinedKeys"] = json_out["config"]["iiif_url"] +
"/" +
json_out["data"][0]["imageID"] +
"/full/!320,240/0/default.jpg"`

That would leave me free to initialize the PyPortal object with

`image_json_path = ["joindKeys"],`

I can imagine other use cases for accessing or modifying the JSON/dict data immediately after its creation.

Thank you

Start by tracing PyPortal.fetch and the existing json_transform handling, then inspect where image_json_path and json_path are resolved. Done means the transform can modify the fetched JSON/dict before either path is used, including constructing the requested image URL from multiple values.

Written by the indexing model from the issue text.