Rewrite download manager example

tidelyrequested to merge
tidely/arti:main into main

Rewrite the gsoc 2023 download-manager example. Fixed #1471 (closed) and partly fixes #1386

There are currently still some licensing issues and the example still needs to be added to the /maint/matrix-check python script.

I'm looking for a certificate store crate that would have a compatible license, if somebody knows one let me know.

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