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/core/tests/unit_tests/messages/test_imports.py

@@ -21,6 +21,7 @@ EXPECTED_ALL = [
     "ToolCallChunk",
     "ToolMessage",
     "ToolMessageChunk",
+    "RemoveMessage",
     "convert_to_messages",
     "get_buffer_string",
     "merge_content",