Move parsing of init args in CKE to __post_init__.

This enables users to add additional fields to a custom ChunkKeyEncoding
without having to override __init__ and taking care of immutability of
the attrs.

Author: RFLeijenaar

src/zarr/core/chunk_key_encodings.py

@@ -33,9 +33,8 @@ class ChunkKeyEncoding(Metadata):
     name: str
     separator: SeparatorLiteral = "."
 
-    def __init__(self, *, separator: SeparatorLiteral) -> None:
-        separator_parsed = parse_separator(separator)
-
+    def __post_init__(self) -> None:
+        separator_parsed = parse_separator(self.separator)
         object.__setattr__(self, "separator", separator_parsed)
 
     @classmethod