Backend: Duo RCA usage rate

Overview

This issue is to add the PostgreSQL version of the RCA usage rate metric i.e. total unique Duo users in a namespace who used RCA / total seats assigned. The metric should be based off of the events in the ai_usage_counts table and be exposed via the AiMetricsBasic endpoint.

This metric will be added to the table which is being introduced in Frontend: PG Duo usage metrics table first iter... (#578196) • Unassigned • 18.7