backend, frontend: basic build tagging
    
    We don't need a per-build tagging mechanism ATM, but we need
    per-mock-chroot tag propagated to builds, as we have two kinds of the
    ppc64le builders.  Power8 machines (might be used for building Fedora
    and older Enterprise Linux chroots), and Power9 (can be used for
    Enterprise Linux 9+, but also other/older chroots).
    
    The point here is to never assign Power8 machine for the EL 9 chroot
    build task.  For this, we need to have a special tag (say Power9, or
    Power9_needed) and a correspondingly tagged Resalloc resources.
    
    Also, while we have separate Resalloc pools for the Power8 and Power9
    architectures, we need to be able to appropriately set the limits for
    each of those; therefore there's the new `builds_max_workers_tag`
    option.
    
    Merges: #2032