Prior to this commit, MVC function endpoints would allow Server Sent
Event responses through `ServerResponse.sse()`. While this covers a
common use case for streaming responses, other technologies would
benefit from a "low-level", unopinionated streaming support.
This commit introduces a new `BodyBuilder.stream()` methods that enables
such use cases. Developers are in charge of setting the relevant HTTP
response headers beforehand, and then can write to the response as raw
`String`, `byte[]` or using complex objects and the configured message
converters for serialization.
Because each streaming protocol has different message separator
semantics, it is also the developers' responsibility to flush buffered
content to the network once a message has been fully written.
Closes gh-32710
2b9b581024