feat: add Confluent Cloud billing plugin

[LZG3530606141]

Summary

Implements Confluent Cloud billing plugin for OpenCost Plugins, as requested in #47.

What

• New ConfluentCostSource plugin that ingests Confluent Cloud billing data into OpenCost
• Follows the OpenCost plugin architecture (hashicorp go-plugin + gRPC)

Features

• Cost listing: Fetches costs via Confluent Billing v1 API with pagination support
• Basic Auth: Uses API Key + API Secret with HTTP Basic authentication
• Rich cost mapping: Maps Confluent cost fields to FOCUS spec including:
  • Product (Kafka, Schema Registry, ksqlDB, Flink, etc.)
  • Line Type, Resource Display Name, Environment
  • Network Access Type, Granularity
  • Amount, Original Amount, Discount Amount
  • Quantity, Unit, Price
• Rate limiting: Respects Confluent Cloud API rate limits
• Pagination: Handles paginated API responses via page_token

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/confluentconfig.go - Configuration struct
  • confluentplugin/confluentbilling.go - API response types
• Config: confluent_api_key (required), confluent_api_secret (required), confluent_plugin_log_level (optional)
• CostSource: "data_streaming", Domain: "confluent"

Testing

Unit tests cover:

• Confluent Cost JSON unmarshalling with nested resource/environment
• CostList paginated response parsing
• Multiple product types (Kafka, Schema Registry)

References

• Confluent Billing API: https://docs.confluent.io/cloud/current/api.html#tag/Costs-(billingv1)
• Confluent Go SDK: https://pkg.go.dev/github.com/confluentinc/ccloud-sdk-go-v2/billing/v1
• Example PR: Atlas plugin with DCO commit  #41
• Bounty program: https://www.opencost.io/blog/introducing-opencost-plugins
• Issue: Confluent Plugin #47

Closes #47

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