add stripe.util.populate_headers (#6637)

Author: asottile

stubs/stripe/stripe/util.pyi

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, overload
 
 def utf8(value): ...
 def log_debug(message, **params) -> None: ...
@@ -12,3 +12,8 @@ class class_method_variant:
     method: Any
     def __call__(self, method): ...
     def __get__(self, obj, objtype: Any | None = ...): ...
+
+@overload
+def populate_headers(idempotency_key: None) -> None: ...
+@overload
+def populate_headers(idempotency_key: str) -> dict[str, str]: ...