ActiveStorage: Using upload_preset with folder creates invalid image links

## Describe the bug in a sentence or two.
I use different upload presets in my app which default to a different folder each (e.g. "production").
storage.yml:
```
cloudinary:
service: Cloudinary
upload_preset: <%= ENV['CLOUDINARY_UPLOAD_PRESET'] %>
```
What the SDK cannot know is that the upload preset is configured to upload files into a specified folder. Therefore, when it generates links to the uploaded images, they miss the folder part and result in dead links.

## Issue Type
[ ] Build - Can’t install or import the SDK
[ ] Performance - Performance issues
[X] Behaviour - Functions aren’t working as expected (Such as generate URL)
[ ] Documentation - Inconsistency between the docs and behaviour
[ ] Other (Specify)

## Steps to reproduce
1. Configure an upload preset using an upload folder (e.g. "test") in the Cloudinary administration interface.
2. Configure Rails' storage.yml to use `upload_preset: test` with the configured cloudinary service (see above).
3. Upload a file attachment via Rails and ActiveStorage, then use url_for() to generate a link to that image.
4. Observe: the link misses the "test"-folder part.

## Operating System
[ ] Linux
[ ] Windows
[ ] OSX
[X] All

## Environment and Libraries
Cloudinary Ruby SDK version - 1.13.2
Ruby Version - 2.6.6
Rails Version - 5.2.4.1
ActiveStorage - 5.2.4.1

## Suggestion
We could use the Cloudinary API to fetch the configured upload folder for the specified upload preset. I'm not sure that's a good idea - we might end up calling the API every time someone uses an upload_prefix.
We may also introduce another storage.yml option "preset_folder" to be used together with upload_preset only.

I need this fixed quite urgently so I'm absolutely willing to submit a PR. Any hints from the maintainers would be totally helpful!