Get the last updated timestamp for the Time-of-Use forecasts for the grid connection point.

The Time-of-Use (ToU) optimization runs on a given resolution of 15 minutes.
For each time segment, i.e. [10:00, 10:15] it publishes a series of forecasts for
the grid connection point associated with a system with systemID.

This endpoint requires to specify a time window called interval for which the forecasts are returned.
For example [2021-01-01T02:07:14Z, 2021-01-02T02:07:14Z].
If no interval is provided it is assumed to be [00:00:00 today, 00:00:00 in two days].
The maximum size of this interval is 48h.

This endpoint returns the last timestamp RFC3339 format when the forecasts in the selected time window
were updated.