Add combine_dicts function and CombineColumns class

[alvarobartt]

Description

This PR adds the combine_dicts function to combine the columns of *StepInput into a single one i.e. the result is a Python dictionary where each key contains a list as a value.

Additionally, this PR also includes the CombineColumns class which is a class inheriting from Step which implements the process method that calls the combine_dicts function. Also the inputs and ouputs properties are defined by the attributes merge_columns and output_merge_columns, respectively.

Example

from distilabel.pipeline.step.combine import CombineColumns

step = CombineColumns(
    merge_columns=["generation", "model_name"],
    output_merge_columns=["generations", "model_names"],
)