The ColumnInfo structure should live in the estimators, rather than transformers

[sfilipi]

For OneHotEncoding estimator/transformer, ColumnInfo lives in the estimator.
For most other pairs, ColumnInfo lives in the transformer.

Standardize their location.

I think they should live in the estimator, since they are mostly used on Estimator definition.

[sfilipi]

@Zruty0, @CESARDELATORRE moving them to the estimator sounds good? it would be another breaking change.

[TomFinley]

Hi @sfilipi I just ran into this wile trying to use ValueToKeyMappingEstimator... it's ColumnInfo is for some reason off in the transformer, not the estimator. Yes should fix.

[artidoro]

We have completed this work for all public ColumnInfo objects.