feat: 891 - new "uploaded timestamp" field for raw images

[monsieurtanuki]

What

• Now we use the "uploaded timestamp" field that the server provided for raw images, but that we just didn't retrieve.
• With this field we'll be able to say if an uploaded image is "too old", e.g. if it was uploaded more than a year ago.

Fixes bug(s)

• Add field "uploaded timestamp" for "raw" images #891

Part of

• Mobile app: fine-tuning needed for 'image_to_update' openfoodfacts-server#9226
• New feature: create a screen to view all photos from a product smooth-app#4674

Impacted files:

• api_json_to_from_test.dart: added tests around new field uploaded
• json_helper.dart: added the uploaded field to from/to json conversion methods; refactored
• product_image.dart: added field uploaded, for uploaded images only

[AshAman999]

I think we can have a better variable name, maybe something like uploaded_on or created_at. Or is it the same var name in other places?

[monsieurtanuki]

Hi @AshAman999!

In this PR we have an 'uploaded_t' JSON tag and an int? uploaded field.

If we compare to similar fields:

• we also have a 'created_t' JSON tag and a created field (cf. in Product)
• but instead of int? in this PR, we do have a much more explicit DateTime?

Would you be OK with DateTime? uploaded? That would be consistent with the rest of the timestamp fields in the package, and wouldn't be confusing with a DateTime? type.

[teolemon]

per @AshAman999 's comment

[monsieurtanuki]

Thank you @teolemon @AshAman999 for your comments!
I don't merge right now as DateTime? uploaded was indeed a good idea instead of int?. I'll implement that by tomorrow.

[monsieurtanuki]

Thank you @teolemon @AshAman999: I've just merged the code with DateTime? uploaded field.