Replace reqwest with ureq

[swsnr]

reqwest is async first and thus brings in a lot of dependencies for the async ecosystem which we effectively don't need since we only use the blocking interface.

ureq only does blocking IO, and has a lot less dependencies.

Unfortunately, it's HTTP proxy support is quite sub-par (see algesten/ureq#668), as it only supports a single static proxy per connection pool which is nowhere near adequate for real world use in general, but for a simple command line tool we can probably ignore this.

[swsnr]

Closing in favour of #304