Update interval metrics to work with arbitrary interval bounds

[brsnw250]

🚀 Feature Request

Add support for Coverage and Width to handle arbitrary interval bounds (quantiles, upper/lower bounds)

Proposal

For Coverage and Width:

1. Add upper_name: Optional[str] = None and lower_name: Optional[str] = None to __init__
2. Raise error if only one border name set
3. Check, that intervals presented in y_pred.prediction_intervals_names
4. Obtain intervals from y_pred using get_prediction_intervals method and estimate metrics
5. Raise error if quantiles and name pair (upper_name, lower_name) are both set and point to different intervals in the prediction dataset

Test cases

1. Test expected metrics values produced
2. Test metrics able to handle different intervals (quantiles, upper/lower bounds)

Additional context

No response