Support string time series for events

[hulien22]

Would be useful to have string timeseries be queryable and be able to be shown as events/annotations. For example, having a string timeseries for a valve that has "on" and "off" values.

Some things to consider:

• Having too many annotation slows down grafana a lot, so this may be a concern here
• We might only want to show some of the points, ie where the values change
• Might potentially want this to be applicable to normal time series as well (eg 0,1 timeseries?)