[PySpark] - Add partial support for window function

[mariotaddeucci]

Initial support for window functions by introducing a PySpark-like Window API and a WindowSpec class.

The implementation is partial: partitioning, basic ordering-by-names, and frame specification (rowsBetween / rangeBetween) are supported and converted to SQL window clauses, but extracting ordering direction from Column expressions is not yet implemented (see the TODO in WindowSpec._columns_as_str). Using Column expressions in orderBy currently raises a ContributionsAcceptedError.

Additionally, a set of functions was added for convenient use with .over(window):

• row_number
• rank
• dense_rank
• cume_dist
• percent_rank
• lag
• lead
• nth_value

[mariotaddeucci]

I don't know how but the next step is to implement proper handling of Column expressions in ordering (including asc/desc).

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