text: Title: Add supports_multivariate to ForecastingModel
  Body: **Is your feature request related to a current problem? Please describe.**
  It is dificult to determine whether a model supports multivariate time series or only univariate ones without building a dictionary of this.
  
  **Describe proposed solution**
  `ForecastingModel` gets a new attribute `supports_multivariate`. This is in line with `supports_past_covariates` etc...
  
  **Describe potential alternatives**
  Model via a superclass. But this would complicate things unnessesarily.
  
  **Additional context**
  This is cool if iterating over multiple models and evaluating their performance (semi-)automatically. This would allow to generate table contents similar to [this](https://unit8co.github.io/darts/index.html#forecasting-models) when reporting on models behaviour (error, ...).