`arguments` field of `ChatFunctionCall` should be `Text` rather than `Value`

The arguments field of a function call object is not guaranteed to be valid JSON, here's the relevant quote from the API reference
```
The arguments to call the function with, as generated by the model in JSON format. Note that the model does not always generate valid JSON, and may hallucinate parameters not defined by your function schema. Validate the arguments in your code before calling your function.
```

Currently if there is a problem in `arguments` we get a `DecodeFailure ... :: ClientError` from `completeChat`, which makes it harder to recover from, because we do not have access to the other parts of the response, which are likely valid.