Function default_power_levels_content 

fn default_power_levels_content(
    version_rules: &RoomVersionRules,
    power_level_content_override: Option<&Raw<RoomPowerLevelsContentOverride>>,
    preset: &RoomPreset,
    users: BTreeMap<OwnedUserId, Int>,
) -> Result<Value>

creates the power_levels_content for the PDU builder