Add API endpoint to fetch billing and usage data

Description:
Right now, the only way to see monthly spend and cost breakdowns is through the Groq Console UI. This works fine for manual checks, but it’s a bottleneck for automation.

An API endpoint that returns billing/usage data (similar to what’s displayed under Dashboard → Usage → Cost) would allow developers to:

  • Programmatically track spend in real time

  • Trigger alerts when spend exceeds a threshold

  • Integrate Groq usage into internal cost dashboards or budget systems

Requested features:

  • REST endpoint returning JSON with monthly spend, daily spend, and per-model usage

  • Support for date ranges

  • Same update frequency as the current console view (~15 min delay is fine)

This would save developers from having to scrape the UI and enable better spend management for production workloads.