Implement API subscriptions for api-platform

[CrowleyRajapakse]

Purpose

Implement API subscriptions for api-platform

Summary by CodeRabbit

• New Features

  • Subscription plans management with configurable throttling/quota
  • API-level subscriptions with token-based access and optional app fallback
  • Subscription validation policy enforcing active subscriptions and per-subscription rate limits
  • REST APIs to create/list/get/update/delete subscriptions and plans
  • Optional encryption for subscription tokens; API configs can list available plans

• Documentation

  • Detailed design, implementation plan, and API usage examples added