Make mutation_token optional in LinkedItem

Closes #35

Co-authored-by: SiJing Zheng <fulminatingmoat@fulminatingmoat.com>

mauigpapi/types/thread_item.py

@@ -408,7 +408,7 @@ class LinkItem(SerializableAttrs):
     text: str
     link_context: LinkContext
     client_context: str
-    mutation_token: str
+    mutation_token: Optional[str] = None
 
 
 class ReelShareType(SerializableEnum):