Example of dynamically generating a zip file download from ASP.NET Core, with minimal threading and memory usage.

Specifically, with this code:

• No file is ever completely in memory at any time. As files are read, they are immediately compressed and sent to the browser.
• No threads are ever blocked waiting on reads or writes. The fully-asynchronous code permits threads to return to the thread pool unless they are actually executing code.

This example uses its own FileCallbackResult to stream output on demand, and includes a workaround for a bug in the ZipArchive class.