Entrypoints for RobustScaler, ToString removed

[ganik]

ML.NET 1.5.1 breaking changes for NimbusML.
Entrypoints RobustScaler, ToString and CategoryImputer removed.
The PR was #5209
NimbusML can do changes for CategoryImputer to use ReplaceMissingValues entrypoint
We need to expose TypeConvertingTransformer and NormalizingEstimator as entrypoints

[michaelgsharp]

I'll work on that today Gani. I thought they were already exposed.

[michaelgsharp]

@ganik @harishsk, TypeConvertingTransformer is exposed as Transforms.ColumnTypeConverter, it is already there.

NormalizingEstimator was exposed as different entrypoints for each type of normalizer. Not sure why it was done that way, I thought it was done the same way as the others where it was just one entry point with the different parameters. I'll have a PR out today for that entrypoint fix.