core, partners: add token usage attribute to AIMessage (#21944)

```python
class UsageMetadata(TypedDict):
    """Usage metadata for a message, such as token counts.

    Attributes:
        input_tokens: (int) count of input (or prompt) tokens
        output_tokens: (int) count of output (or completion) tokens
        total_tokens: (int) total token count
    """

    input_tokens: int
    output_tokens: int
    total_tokens: int
```
```python
class AIMessage(BaseMessage):
    ...
    usage_metadata: Optional[UsageMetadata] = None
    """If provided, token usage information associated with the message."""
    ...
```

libs/partners/mistralai/tests/integration_tests/test_standard.py

@@ -20,3 +20,14 @@ class TestMistralStandard(ChatModelIntegrationTests):
             "model": "mistral-large-latest",
             "temperature": 0,
         }
+
+    @pytest.mark.xfail(reason="Not implemented.")
+    def test_usage_metadata(
+        self,
+        chat_model_class: Type[BaseChatModel],
+        chat_model_params: dict,
+    ) -> None:
+        super().test_usage_metadata(
+            chat_model_class,
+            chat_model_params,
+        )