AI Impact: Expose monthly Duo Chat usage data

Summary

As per AI Impact: collect Duo chat usage data (#459967 - closed) we are collecting Duo Chat usage data for AI Impact: Add GitLab Duo seats tile (#460832)

In this issue we need to aggregate data and expose it to FE via GraphQL

Proposal

• Create new fields under aiMetrics GraphQL type which represent Duo Chat seats assigned and used.
• Definition of "used" - user did at least 1 prompt to the chat during given time period.