feat!: tolerate index field in stages and attachments

[adubovik]

Resolves #293

Breaking changes

The required index field has been removed from the ToolCall class in the chat completion request (note that in the responses it's still there).
It's not needed in the chat completion request anyway, since each tool call has a definite position in the list of tool calls.

Migration

1. The migration at the ToolCall construction places - remove the index field:

tool_call = ToolCall(
-    index=tool_call_index,
    id="tool-call-id",
    type="function",
    function=function
)

2. The migration at the ToolCall.index access points - assume the index field is always None:

- if tool_call.index is None:
    # handle None case
- else:
-   # handle not None case