delete method
deletes a file from the server(s) 
if serverUrls is null, the userServerList is fetched from nostr. \
Implementation
Future<List<BlobDeleteResult>> delete({
  required String sha256,
  List<String>? serverUrls,
}) {
  return _blossom.deleteBlob(
    sha256: sha256,
    serverUrls: serverUrls,
  );
}