feature: Add OpenAI conversation id

The AI Agent node lacks the option to pass the "conversation" id with the request, which prevents the use of the OpenAI Responses API in stateful mode. A dedicated field for this id would be nice, or even a more general way of adding arbitrary application-defined elements to the request.

+The way function call responses are delivered must be adjusted in stateful mode, as currently the initial user prompt appears to be passed back to the LLM every time along with the function result, not only creating duplicates in the server side conversation context, but potentially causing the model to call the function again in a self perpetuating loop.