sources/ldap: add list_flatten function to property mappings, enable on managed LDAP mappings

closes #2199

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/expressions/_functions.md

@@ -18,6 +18,17 @@ Example:
 user_email_local = regex_replace(request.user.email, '(.+)@.+', '')
 ```
 
+### `list_flatten(value: list[Any] | Any) -> Optional[Any}`
+
+Flatten a list by either returning its first element, None if the list is empty, or the passed in object if its not a list.
+
+Example:
+
+```python
+user = list_flatten(["foo"])
+# user = "foo"
+```
+
 ### `ak_is_group_member(user: User, **group_filters) -> bool`
 
 Check if `user` is member of a group matching `**group_filters`.