Make createFile method public

Problem/Motivation

This is not a bug per se, but something to be improved to allow other modules reuse code (for example to create Cloudinary files as part of a migration process)

createFile() method is duplicated in cloudinary_media_library_widget\Plugin\CloudinaryMedia\CloudinaryFile and cloudinary_media_library_widget\Plugin\CloudinaryMedia\CloudinaryImage plugins, and it has protected visibility, so it can't be reused.

Steps to reproduce

Proposed resolution

Move createFile() method to CloudinaryMediaBase class, which is extended by CloudinaryMedia plugins, and make it public.

Remaining tasks

User interface changes

API changes

Data model changes