feat: add Fastly billing plugin

[LZG3530606141]

Summary

Implements Fastly billing plugin for OpenCost Plugins, as requested in #48.

What

• New FastlyCostSource plugin that ingests Fastly CDN billing data into OpenCost
• Follows the OpenCost plugin architecture (hashicorp go-plugin + gRPC)

Features

• Invoice listing: Fetches invoices via Fastly Billing v3 API with pagination support
• Invoice details: Retrieves detailed line items for each invoice
• Rich cost mapping: Maps Fastly line items to FOCUS spec fields including:
  • Product Group, Product Line, Product Name
  • Usage Type, Region, Units, Rate
  • Billed Cost, List Cost, List Unit Price
• Rate limiting: Respects Fastly API rate limits (1.5 req/s)
• API Key authentication: Uses Fastly-Key header for authentication

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/fastlyconfig.go - Configuration struct
  • fastlyplugin/fastlybilling.go - API response types
• Config: fastly_api_key (required), fastly_plugin_log_level (optional)
• CostSource: "CDN", Domain: "fastly"

Testing

Unit tests cover:

• Fastly API JSON response unmarshalling
• Invoice and transaction line item parsing
• API key header verification via httptest mock server

References

• Fastly Billing API: https://www.fastly.com/documentation/reference/api/account/billing/
• Example PR: Atlas plugin with DCO commit  #41
• Bounty program: https://www.opencost.io/blog/introducing-opencost-plugins
• Issue: Fastly Plugin #48

Closes #48

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