Add Additional Aggregation Functions

[bcodell]

Description

It's technically outside of the Activity Schema spec, but it would be nice if the package shipped with additional aggregation functions to use with the aggregate relationships. Aggregations include:

• average
• median
• listagg (available in Narrator)
• listagg distinct
• count distinct
• boolean sum - convert boolean features to integers and sum them
• not null - returns true if the feature has at least one non-null value, else false. Useful for sparse features.

Dependencies

• Activity Registry #25 - dbt project needs to know data types for features
• Add Aggregation Registry #28 - new aggregations need to be registered in the Aggregation Registry

Implementation

• Add a macro for each aggregation with the naming convention _aggfunc_name.sql (e.g. _average.sql)
• Implement each using the caller() implementation pattern (see example)
• Register the aggregation in the Aggregation Registry

Checklist for each of the aggregations to implement:

• average
• median
• listagg
• listagg distinct
• count distinct
• boolean sum
• not null

Open Questions

• Are these reasonable to implement, even though they aren't included in the Activity Schema spec?
• Do the aggregations specified make sense?
• What other aggregations should be added?