Enable cross-validation for LearningPipeline

[GalOshri]

Cross-validation would make it easier to validate models if the data is not already split into train and test files (and generally helps give more precise metrics). This is already exposed as an entrypoint here, but needs to be enabled for LearningPipeline.

[codemzs]

@GalOshri This is more than enablement, here we need to extend the entrypoint to support models of type ITransformModel because currently entrypoint for CV only supports models of IPredictorModel and the learning pipeline only produces models of type ITransformModel. Please refer to PR #212 .