Add SSE (Server-Sent Events) Support to QtHttpServer

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events

Server-Sent Events (SSE) support in QtHttpServer allows unidirectional, real-time communication. SSE enables continuous server-to-client updates without requiring WebSockets. This feature is essential for protocols like the Model Context Protocol, which rely on a persistent data stream from the server.

https://modelcontextprotocol.io/introduction