Add masked backtest

[alex-hse-repository]

🚀 Feature Request

Add ability to set masks for folds in backtest

Motivation

More flexibility for backtest strategy

Proposal

1. Add class FoldMask(see etna-utils)
2. In AbstractPipeline

• change the signature of backtest: n_folds: Union[int, List[FoldMask]] = 5

3. In BasePipeline

• change the signature of backtest: n_folds: Union[int, List[FoldMask]] = 5
• use the backtest implementation from etna-utils
• if n_folds is int, create fold masks for the n_folds

4. In all the places where backtest is used make n_folds: Union[int, List[FoldMask]]

Test cases

Add tests from etna-utils

Alternatives

No response

Additional context

Blocked by #562

Checklist

• I discussed this issue with ETNA Team