v2.1.0 — Integrate official Amazon Creators API PHP SDK

Added

• Official Amazon Creators API PHP SDK (v1.2.0) — replaces custom HTTP/OAuth with Amazon's typed SDK classes
• Composer Dependency Management — Guzzle HTTP 7.x, PSR-7, PSR-HTTP-Client via autoloader

Changed

• Amazon_API class rewritten to use SDK DefaultApi, typed request models, and resource enum constants
• Product model from_api_response() updated to accept SDK Item objects with typed getters
• Minimum PHP version raised from 7.4 to 8.1 (SDK requirement)

Removed

• RequestSigner class — SDK handles OAuth 2.0 token management internally
• Marketplace::get_token_endpoint(), Marketplace::get_endpoint(), Marketplace::version_to_region() — SDK manages endpoints internally