Allow specifying request body in SSE request

[allantl]

Currently, SSE implementation does not support specifying a request body. Would be very helpful if this is implemented since most GenAI APIs needs a POST request containing a JSON payload with model parameters and input text.

[ryber]

Just a note, I am looking into supporting it but SSE does not actually support bodies at all and the reference Javascript class is incapable of doing it. The server framework this project uses to test (Javalin) also does not support bodies. It doesn't appear there is any technical reason for it to be like this. It was just assumed it wouldn't be needed. So I'm curious why an LLM would "need" this to be supported when the entire feature is obviously hostile to the idea.