- Add hmac, sha2, and base64 dependencies to Cargo.toml for JWT support
- Add README.md with installation instructions, architecture overview, and API examples for all SDK modules
- Add AuthProvider class with API key and JWT token support (generate_jwt_token, validate_jwt_token, get_headers, clear_token_cache methods)
- Add HTTPClient base class with retry logic, error handling, and timeout management
- Add SubscriberAPI module
