Feature: download key instead of storing it locally in this repository

[maertsen]

We could download the key instead of storing it, to avoid having it in two places.

Fetching could be done by a "build.rs" Rust "script" in the root of the project dir. See: https://doc.rust-lang.org/cargo/reference/build-scripts.html and https://stackoverflow.com/a/45623133.

This would need proper error handling and validation, to make sure we are shipping the right key. So the gains over local storage may be small.

Deferring for now.