Add annotation-regressors to forecaster config

[Flix6x]

Inspired by Ascension Thursday 2026, which saw a morning price peak forecast that is more typical of ordinary week days. On sunny weekend days and public holidays, we more typically observe monotonically declining prices from midnight to noon.

(source)

I am moving holiday regressors into the FlexMeasures forecasting tooling, but want to do so via the annotations model. My reasons:

1. This allows other types of annotations to serve as regressors, too. For instance, personal holidays (residential), busy periods (logistics), and planned shutdowns (industry).
2. This improves traceability and reproducibility of forecasts, because annotations are stored as time series themselves.