fields-azure: Add support for preserve storage config parameter

[sfodor]

The preserve storage config parameter already exists in Keystone, see https://keystonejs.com/docs/config/config#storage-images-and-files. Currently the files are not deleted when they are removed from Keystone's database. It means wasted storage space, and possible unwanted files in azure blob storage.

[gautamsi]

this is done, let me know if this worked.

[sfodor]

Thanks. It works.