Feat: Allow popper to receive zIndex from user settings

[anhvy]

By default, react-tippy set tippy-popper's z-index to 9999, this change will allow user to pass their own value.
Example:

<Tooltip
  useContext
  position="bottom-end"
  trigger="manual"
  theme="dark-info"
  html={<NotificationList closeActivityFeed={closeActivityFeed} />}
  interactive
  arrow
  arrowSize="big"
  distance={15}
  offset={30}
  open={tooltipOpenState}
  onRequestClose={closeActivityFeed}
  zIndex={10000}
>
  <div style={styles.tooltipTrigger} onClick={openActivityFeed}>
    <Icon name="ui-1_bell-53" style={styles.bellIcon} />
    {counter > 0 && (
      <Badge type="danger" text={counter} style={styles.countBadge} />
    )}
  </div>
</Tooltip>