Fix the tool_choice format for named choice by adapting OpenAIs scheme

[linusbierhoff]

This pull request resolves an issue with the ToolChoice/ToolType struct.
Currently, the API does not allow the OpenAI tool_choice schemes. Currently, the API expects this format to force the usage of an explicit tool:

"tool_choice": {
    "Function": {
         "name": "function_name"
    }
}

(or "tool_choice": “function_name”). Both are incompatible with the OpenAI format.

This pull request changes two things:

1. Keyword “Function” → “function”
2. adding the OpenAI schema for named choices:

"tool_choice": {
    "type": "function",
    "function": {
         "name": "function_name"
    }
}

@Narsil