core: add RemoveMessage (#23636)

This change adds a new message type `RemoveMessage`. This will enable
`langgraph` users to manually modify graph state (or have the graph
nodes modify the state) to remove messages by `id`

Examples:

* allow users to delete messages from state by calling

```python
graph.update_state(config, values=[RemoveMessage(id=state.values[-1].id)])
```

* allow nodes to delete messages

```python
graph.add_node("delete_messages", lambda state: [RemoveMessage(id=state[-1].id)])
```

libs/community/tests/unit_tests/load/test_serializable.py

@@ -95,6 +95,13 @@ def test_serializable_mapping() -> None:
             "structured",
             "StructuredPrompt",
         ),
+        # This is not exported from langchain, only langchain_core
+        ("langchain", "schema", "messages", "RemoveMessage"): (
+            "langchain_core",
+            "messages",
+            "modifier",
+            "RemoveMessage",
+        ),
     }
     serializable_modules = import_all_modules("langchain")