feat: add GitHub Billing plugin

[LZG3530606141]

Summary

Implements GitHub Billing plugin for OpenCost Plugins, as requested in #42.

What

• New GitHubBillingCostSource plugin that ingests GitHub billing data into OpenCost
• Follows the OpenCost plugin architecture (hashicorp go-plugin + gRPC)

Features

• Usage billing: Fetches standard usage via /organizations/{org}/settings/billing/usage
• Premium request usage: Fetches Copilot and premium API usage via /organizations/{org}/settings/billing/premium_request/usage
• Bearer token authentication: Uses GitHub personal access token
• Rich cost mapping: Maps GitHub billing fields to FOCUS spec including:
  • Product (Actions, Packages, Copilot, etc.)
  • SKU, Repository Name, Organization
  • Unit Type, Price Per Unit
  • Gross Amount, Net Amount, Discount Amount
  • Model (for premium/Copilot usage)
• Rate limiting: Respects GitHub API rate limits (5000 req/hr)
• Year/month filtering: Queries billing data by time window

Implementation Details

• Plugin directory structure follows OpenCost conventions:
  • cmd/main/main.go - Main plugin implementation
  • cmd/main/main_test.go - Unit tests
  • cmd/validator/main/main.go - Config validator
  • config/githubbillingconfig.go - Configuration struct
  • githubbillingplugin/githubbilling.go - API response types
• Config: github_api_token (required), github_organization (required), github_billing_plugin_log_level (optional)
• CostSource: "dev_tools", Domain: "github"

Testing

Unit tests cover:

• Standard usage item JSON unmarshalling
• Premium usage item JSON unmarshalling
• Usage response with multiple items (Actions, Packages)

References

• GitHub Billing API: https://docs.github.com/en/rest/billing/usage
• Example PR: Atlas plugin with DCO commit  #41
• Bounty program: https://www.opencost.io/blog/introducing-opencost-plugins
• Issue: Github Billing and Usage #42

Closes #42

Signed-off-by: LZG3530606141 3530606141@qq.com