Add forecast_horizon_minutes to /solar/GB/gsp/forecast/all/

[braddf]

Detailed Description

In order to request N-hour forecasts for multiple GSPs at once; at the moment, you can only request single GSP forecasts with a forecast horizon.

Context

e.g. to grab relevant GSPs for a DNO region in the Quartz Solar UI and aggregate them for the chart.

Possible Implementation

As well as the additional param and logic, will require a change to ocf-datamodel