Add Copilot app metrics fields to Copilot usage metrics structs

[Tens1des]

The Copilot usage metrics API includes used_copilot_app, used_copilot_cloud_agent, totals_by_copilot_app, and daily_active_copilot_app_users, but go-github does not model them yet.

Docs: https://docs.github.com/en/copilot/reference/copilot-usage-metrics/copilot-usage-metrics

Affected structs:

• CopilotDailyMetrics
• CopilotUserDailyMetrics
• CopilotUserPeriodicMetrics

[Tens1des]

I'll take this.