Added an option to download a xapk and made apk the default downloaded type

[Tutez64]

Hi, I think this would make a really great addition. This is my first time doing Rust, I hope it's not too badly made (I reused the code of NoelJacob as a base https://github.com/EFForg/apkeep/commit/0465a26e6de9db9700ac5db094fd3d4bbc9c62b0)