feat: Add dbus API and CLI for battery charge thresholds
    
    The dbus API authenticates with Polkit, which should prompt for an
    administrator password, or run without prompt when the client in root.
    
    Thresholds can be displayed in the CLI with `system76-power
    charge-thresholds`, and set with `system76-power charge-thresholds
    <start> <end>`.
    
    This is currently limited to System76 hardware with open firmware, to
    avoid worrying for now about things like systems with multiple
    batteries, only having an end thresholds, and other differences between
    implementations.