[Storage] Add type signature to readall (Azure#22963)

Author: José Duarte

sdk/storage/azure-storage-blob/azure/storage/blob/_download.py

@@ -510,6 +510,7 @@ def chunks(self):
             chunk_size=self._config.max_chunk_get_size)
 
     def readall(self):
+        # type: () -> Union[bytes, str]
         """Download the contents of this blob.
 
         This operation is blocking until all data is downloaded.