Transform knows if it converts data to regressor

[julia-shenshina]

🚀 Feature Request

Now each transform knows that it can transform the column to regressor but does not contain the info directly. In this issue we need to change it.

Motivation

New way of work with regressors.

Proposal

Create empty mixin FutureMixin

For each transform

• add FutureMixin to the list of ancestors, if the transform can convert non-regressor column to a regressor one
• change work with output_column:
  • if output_column is None, set output_column as transform.__repr__ without any prefixes

Test cases

Update tests for output_column names for all the transforms

Alternatives

No response

Additional context

No response

Checklist

• I discussed this issue with ETNA Team