Better ChatCompletionFunctionCall interface

[m1guelpf]

According to OpenAI's docs, the function_call parameter can only be either none, auto, or { "name": function_name }.

The current interface in this library feels counter-intuitive, and I've often tried to call a custom function using ChatCompletionFunctionCall::String(function_name) instead of ChatCompletionFunctionCall::Object(json!({ "name": custom_function })).

This change proposes a more intuitive interface for this (as a breaking change unfortunately).

[64bit]

Actually previous From conversion trait implemented lets you provide function_call parameter without even using ChatCompletionFunctionCall:

CreateChatCompletionRequestArgs::default()
    .function_call("auto") //  or "none" 

OR

CreateChatCompletionRequestArgs::default()
    .function_call(json!({ "name" : "hello_world" })

---

But yes enum updated in this PR does offer better API.

No worries about breaking changes, because next release v0.15.0 will have other breaking change (already in main) and possible breaking updates from latest openapi spec.

I'll merge this PR once spec sync is addressed too for v0.15.0.

[64bit]

Hello @m1guelpf ,

I took liberty to make some changes and kept your idea intact. Essentially use serde instead custom serialization implementation.

Thank you for your patience on this PR!