Prediction intervals visualization

[WinstonDovlatov]

🚀 Feature Request

Add prediction intervals visualization

Motivation

Tool for forecast visualization

Proposal

Enhance method plot_forecast:

1. Add parameter prediction_intervals: bool = False
2. Add parameter quantiles: Optional[Sequence[float]] = None
3. Method should now plot prediction intervals if prediction_intervals=True using the quantiles from forecast_ts
4. If quantiles=None use all quantiles from forecast_ts
5. Use only the quantiles from quantiles that are presented in forecast_ts(throw warning about missing quantiles)

Add picture with visualization example to PR

Test cases

Check the behavior of the method in different cases:

1. Plot all quantiles from forecast_ts
2. Plot specific quantiles from forecast_ts
3. Incorrect parameters(quantiles are not in forecast_ts e.g. )

Alternatives

No response

Additional context

No response

Checklist

• I discussed this issue with ETNA Team