ENH: custom_seqinfo - provide a way for heuristics to extract/add arbitrary value

Just a draft implementation

Author: yarikoptic

heudiconv/dicoms.py

@@ -9,8 +9,7 @@
 from pathlib import Path
 import sys
 import tarfile
-from typing import TYPE_CHECKING, Any, Dict, Hashable, List, NamedTuple, Optional, Union, overload
-from typing_extensions import Protocol
+from typing import TYPE_CHECKING, Any, Dict, List, NamedTuple, Optional, Union, Protocol, cast, overload
 from unittest.mock import patch
 import warnings