Add a `seprarate_dirname_file`name method in core file storage

[Fournet Enzo]

In /files/storage/base.py file, here's some code that separating filename and dirname so here we can add a separate_dirname_filename method that allows us to modify how Django separates the filename from the dirname when storing a file without fully rewriting any method that needs to perform this separation. This can be useful when a file has a double extension, such as .json.sbom or anything similar, and we don't want to break the extension.

Method example :

def seprarate_dirname_filename(self, filename):
        """
        Return a tuple of dirname and filename.
        This method can be overridden to customize how the filename is split.
        """
        return os.path.split(filename)

With this upgrade, we are able to let people override the Storage Class and only rewrite this new method to be sure that Django don't broke some specific file name or extension, that needed.

[Sarah Boyce]

Hi Fournet, the example you gave feels like a duplicate to #23759 which is now fixed.
Can you check whether the customization is still needed based off main? Or give more examples?