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/anthropic/tests/unit_tests/test_chat_models.py

@@ -89,7 +89,16 @@ def test__format_output() -> None:
     )
     expected = ChatResult(
         generations=[
-            ChatGeneration(message=AIMessage("bar")),
+            ChatGeneration(
+                message=AIMessage(
+                    "bar",
+                    usage_metadata={
+                        "input_tokens": 2,
+                        "output_tokens": 1,
+                        "total_tokens": 3,
+                    },
+                )
+            ),
         ],
         llm_output={
             "id": "foo",