Pivot/Unpivot functionality

[jaychia]

Is your feature request related to a problem? Please describe.

Pivot: Converts rows into columns Unpivot : Converts columns into rows

Tasks

• [ ] Pivot with explicit values passed in
• [ ] Pivot where Daft runs a distinct() operation to get the values first
• [ ] Unpivot

See also:

• Spark pivot: https://spark.apache.org/docs/3.2.1/api/python/reference/api/pyspark.sql.GroupedData.pivot.html
• Ibis pivot_wider (or, just pivot in our case): https://ibis-project.org/reference/expression-tables#ibis.expr.types.relations.Table.pivot_wider
• Ibis pivot_longer (or, unpivot in our case): https://ibis-project.org/reference/expression-tables#ibis.expr.types.relations.Table.pivot_longer