[DF] Add support for filtered groupby aggregations

[charlesbluca]

Adds back support for filtered aggregations with FILTER (WHERE ...), which allows us to unxfail test_group_by_filtered and unblocks several related tests in #746 and #759.

[charlesbluca]

A potential issue with new_temporary_column that came to mind while working on this (I say potential because I'm unsure of the behavior of uuid.uuid4()):

Since we're using the table's columns attribute to check that a random column name hasn't been used yet, and we don't actually assign any of these random columns names until several of them have been generated, it is technically possible (though rare) to accidentally assign multiple input / filter columns to the same random backend name, which will certainly cause issues.

Since assign calls are expensive and we ideally want to be adding all required backend columns in a single go, it might make sense to refactor new_temporary_column to instead look at some attribute of the DataContainer or ColumnContainer to check for duplicates, which are both cheaper to update on the fly.

Don't intend to block this PR, but could be worthwhile to open an issue / TODO to handle this down the line.

[ayushdg]

Agreed that moving the check to dc or cc should make things significantly cheaper.
Based on the article here probability of name collision is almost negligible, especially at the scale at which we generate new columns

[codecov-commenter]

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