Made function call streamable

[buraktabn]

Making FunctionCall's arguments Optional allow us to stream the functions. Otherwise, it throws a parsing error.

[64bit]

Hi @buraktabn ,

Thank you for PR! This is a really good update (and bug fix) to make example fully streamable.

To maintain parity with OpenAPI spec this PR requires following changes:

1. Keep existing FunctionCall as it is because it represents non-streaming response : https://github.com/openai/openai-openapi/blob/master/openapi.yaml#L2124-L2136

2. Introduce, lets say, FunctionCallStream with the Option<String> to represent streaming response: https://github.com/openai/openai-openapi/blob/master/openapi.yaml#L2211-L2220

3. Use FunctionCallStream to be the type for function_call field for ChatCompletionStreamResponseDelta struct.

[buraktabn]

Hi @64bit,

Thanks for the review. I've made the changes as you suggested and pushed the commit Let me know if anything else is needed.

[64bit]

This is good!

[64bit]

This PR is released in v0.13.1