should we deprecate fieldExpression and extractionFn field in various Aggregators, DimensionSpecs, Filters etc

they were added before `VirtualColumn` was introduced and I think a better alternative now could be to have a extractionFn based `VirtualColumn` that could be used and expression based virtual column exists already.
That way, all the filter and aggregator implementations don't need to handle them in different places.

Start by reviewing the Aggregators, DimensionSpecs, and Filters that currently expose fieldExpression or extractionFn, then compare them with VirtualColumn and the existing expression-based virtual column. Done means reaching and documenting a project decision on whether these fields can be deprecated and replaced through VirtualColumn without leaving inconsistent handling across implementations.

Written by the indexing model from the issue text.