arti/examples/gsoc2023/download-manager uses arti-hyper

This example, written as part of a GSOC project last year, was never properly integrated into our tree.

It uses arti-hyper, which I am about to remove. So it's about to stop being built in CI.

Options:

1. Retain it in git (perhaps indefinitely)
2. Delete it
3. Fix it up for hyper 1.x
4. Fix up it and all the other gsoc2023 examples and make them a proper part of our examples/docs