[timeseries] Add sample_weight support to TimeSeriesPredictor

## Description
Add an option to customize the sample weight used by time series forecasting metrics.

There are two types of weighting that would be interesting:
- [x] `horizon_weight`: one weight assigned to each time step in the forecast horizon (planned for v1.3, #5058)
- [ ] `item_weight`: one weight assigned to each item in the dataset (planned for v1.4+)

## References
- [ ] Find examples in other forecasting libraries to find reasonable names in the API.