Provide more ways to stream through ServerResponse for WebMvc functional endpoints

[rstoyanchev]

Currently it's possible to stream SSE events via ServerResponse.sse(Consumer<SseBuilder>), but no way to stream in any other format, comparable to the ResponseBodyEmitter base class of SseEmitter for annotated controller methods.

For extra context, the GraphQL spec is finalizing an incremental delivery feature that prioritizes some parts of the response data and sends them before others. Over HTTP this is expected to be a "mixed/multipart" stream.

• Incremental Delivery WH
• Incremental Delivery over HTTP.