Conditional formatting (color range) on pivot table values to be relative to grouping

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**Is your feature request related to a problem? Please describe.**
I would like to display a color range on table cells based on relative values, but currently the relativity is based on the entire column value range. For instance a pivot table showing customer spend across a range of dates, I would like the conditional highlights to reflect based on spend relative to the customer and not relative across all customers.

**Describe the solution you'd like**
Add an option to make cell highlighting with a color range relative to a grouping or allow a custom function.

**Describe alternatives you've considered**
I've considered trying to create a calculated column myself via a native query with min/max values by grouping, but I cannot use a pivot table visualization using a SQL query.

**How important is this feature to you?**
We are unable to move our largest set of reports to Metabase without this feature.

No files, tests, or entry points are identified in the issue. Start by locating the pivot table visualization and its conditional color-range handling, then determine how grouping should define each range or how a custom function would be configured. Done means grouped relative highlighting works for pivot-table values, with the behavior verified for customer and date groupings.

Written by the indexing model from the issue text.