Download from URL

[pothos]

Currently the ue-rs download needs an XML dump, but we also need to have a mode where an URL can be passed, this is used in the Flatcar postinst hook for old update-engine clients that can't pass the XML dump and the initrd extension download phase (The match string and XML dump arguments are not required/processed then)

Flatcar integration points: flatcar/Flatcar#1028 (comment)

For example, it can be subcommands instead of exclusive arguments:

    if [ "${url}" = "" ]; then
        ue-rs-fetch payload-from-url --payload-url "${url}" --target-file "${target}" --pub-key /usr/share/update_engine/update-payload-key.pub.pem
    else
        ue-rs-fetch payload-from-xml --xml-dump "${from}" --payload-name "${name}" --target-file "${target}" --pub-key /usr/share/update_engine/update-payload-key.pub.pem
    fi