QHttpServer streaming (upload) support

Currently QHttpServer waits for the client upload to be complete before invoking a route handler. This is fine for most use cases, but can become a problem with very large payloads. In these cases it would be beneficial to be able to avoid buffering the entire body to memory first (consider eg. a very large file).

This is task is about creating a mechanism to consume data as it arrives / in some reasonable chunks. Furthermore it should be possible to stop such stream and disconnect the client at will.

Likely this means:

• A route / callback mechanism to trigger as data arrives
• An option / API to abort the connection mid-flight