create CandidatesGenerator

[monkey0head]

create a class to RePlay models for candidates generation, which:

• preprocess log and features
• train models
• generate candidates from each model/selected models
• get fallback prediction and ensemble candidates dataset
• add relevance/rank from each model
• add features from selected models
• add "generated_by" column for each model?

[monkey0head]

fix indexing:
_fit and _predict with external ids filtering (inside scenario) were used by first level models to avoid redundant dataset reindexing. need to consider using _fit_wrap and _predict_wrap without additional ids and seen filtering.