[gcloud] Feature request: allow to upload static files to a specific folder similar to what `STATIC_ROOT` does

[gdalmau]

In https://django-storages.readthedocs.io/en/latest/backends/gcloud.html#getting-started it shows that we can configure STATICFILES_STORAGE = 'storages.backends.gcloud.GoogleCloudStorage' to upload the static files to a Google Cloud Storage bucket, but all the files will be uploaded on the root or on the location specified by GS_LOCATION.

It would be nice to allow to specify a folder to upload the static files in order to differentiate them with the user-uploaded files (media files via the upload_to parameter on FileField).