Allow instance to be passed into the FileField storage callable param

[Viicos]

A while ago #28184 introduced the ability to use callables as the storage parameter of the FileField.

Someone even requested the ability to have the instance passed into the callable: ​https://github.com/django/django/pull/8477/files#r226555624

But I believe @miigotu misinterpreted it and was thinking about a Storage instance.

What could be great is the ability to pass the instance (and maybe the filename too) similar to ​upload_to

Example:

def get_storage(instance, filename):
    if instance.is_private:
        return PrivateStorage()
    else:
        ...

This could allow the user to use a specific storage depending on the instance being saved.