Default usage statistics for streaming responses (#6578)

## Why are these changes needed?

Enables usage statistics for streaming responses by default.

There is a similar bug in the AzureAI client. Theoretically adding the
parameter
```
model_extras={"stream_options": {"include_usage": True}}
```
should fix the problem, but I'm currently unable to test that workflow

## Related issue number

closes https://github.com/microsoft/autogen/issues/6548

## Checks

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<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [ ] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [ ] I've made sure all auto checks have passed.