Update streaming to make use of Rack "callable body" #1892
Description
Instead of adding support for either full or partial hijack, Sinatra should make use of "callable body" (rack/rack#1745) introduced with Rack 3: https://github.com/rack/rack/blob/9eb4aa2646fdaaf22c7f6a9fa4c6dc73e587204f/UPGRADE-GUIDE.md#response-bodies-can-be-used-for-bi-directional-streaming
Originally posted by @dentarg in #604 (comment)