RFE: panels-visualizations/configure-tooltips/: Allow to customize timestamp format

Right now the tooltip [for time series] always shows the related timestamp as `YYYY-mm-dd HH:MM:SS`. Usually this is ok, but sometimes other or additional information like the corresponding abbreviated name of the day of the week would be very helpful. Therefore it would be nice, if grafana would allow one to customize the timestamp format for tooltips as well, e.g. following [strftime(3)](https://man7.org/linux/man-pages/man3/strftime.3.html).