[ENH] Move base class preprocessing functions to utils

[baraline]

Describe the feature or idea you want to propose

To build a hybrid base class (collection in fit and series in predict) for the new BaseSimilaritySearch class, the preprocess_series and preprocess_collection functions need to be moved to utils so they can be reused anywhere.

Describe your proposed solution

Add a file in utils for processing function, exact location TBD (in base or validation or something else ...)

Describe alternatives you've considered, if relevant

No response

Additional context

Making the hybrid base class inherit from both BaseCollection and BaseSeries make no sense, so this is necessary.