Add proxy support to Updater and UpdaterBuilder

[LeeeSe]

Add proxy support to Updater and UpdaterBuilder.

When a binary is packaged as an app and runs on Mac, cargo packager updater doesn't automatically use a proxy to check for and download updates. I've added a proxy configuration option to UpdaterBuilder to enable automatic use of system proxy (optional).

[denjell-crabnebula]

Thanks for the proposal.

I feel at the very least, that this will absolutely need some kind of documentary support, ie. over at https://docs.crabnebula.dev/packager/

[LeeeSe]

Sorry, my description was incorrect. Actually, I just added a proxy parameter for the updater, providing users with an option to set a proxy address for their own app. It's not that I set a switch for the updater that, when turned on, automatically uses the system proxy.

Its usage is as follows:

use cargo_packager_updater::{UpdaterBuilder, Config};
use semver::Version;

let config = Config {
  endpoints: vec!["http://myserver.com/updates".parse().unwrap()],
  pubkey: "<pubkey here>".into(),
  ..Default::default()
};

let current_version = Version::parse("0.1.0").unwrap();

let updater = UpdaterBuilder::new(current_version, config)
    .proxy("http://your-proxy-address:port") // set your proxy url
    .build()
    .unwrap();

if let Some(update) = updater.check().expect("failed while checking for update") {
    update.download_and_install().expect("failed to download and install update");
}

If you think this small change needs to be documented on the doc, I'm willing to do so.