fix: Update OutputItem to align with OpenAI's Specification

[Advayp]

Description

• According to OpenAI's specification, OutputItem should be a tagged union. This PR updates that struct to align with OpenAI's specification
• A nice benefit of this is that streaming with tool calls is now supported

[64bit]

Looks like this PR is updating incorrect type, the Response object output field type is OutputContent

https://github.com/64bit/async-openai/blob/main/async-openai/src/types/responses.rs#L1371

[Advayp]

Looks like this PR is updating incorrect type, the Response object output field type is OutputContent

https://github.com/64bit/async-openai/blob/main/async-openai/src/types/responses.rs#L1371

The OutputContent type is already correct. I'm updating OutputItem because that's the output type used in streaming events. Though, after taking another look now, both OutputContent and OutputItem are the same. Because of this, I think we can remove the OutputItem type entirely and replace it with OutputContent. What do you think?