config: add supported_arches
    
    This allows supported_arches to be configured, which should be used by
    our directives (like koji) to decide which architectures to act upon
    (e.g. which binary RPMs to download). This option defaults to primary
    Fedora architectures.
    
    Koji calls had a special parameter `arches=['all']` which used to
    download all available RPMs. It now downloads all available *supported*
    RPMs (per `config.supported_arches`). This allows dynamic configuration
    on a server to handle only specific architectures (or extend handling
    beyond just primary ones). Tasks can still download "non-supported"
    architectures if they list them explictly.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1038