Replace reqwest with ureq

[emilk]

This removes around 750 lines of dependencies from Cargo.lock (see e9e4b25), making ffmpeg-sidecar compile faster.
The code is also shorter.

I also added Cargo.lock to git, for predictable builds (important for e.g. git bisect, and also to prevent CI from failing just because some 3rd party crate made some change). See https://blog.rust-lang.org/2023/08/29/committing-lockfiles.html for more.

Oh, and thanks for an awesome crate 🙏

[nathanbabcock]

I'm quite happy with the improvement in build times on this PR. ureq looks like a great fit.

Thank you for the nice find and the kind words. Also the tips about library best practices are appreciated too. 😎