Add retry configuration to client constructor

[Nancy-Chauhan]

Currently retry behavior is configured with just max_retries on the client. Users should be able to pass a full RetryConfig to control:\n\n- Base delay\n- Max delay\n- Backoff multiplier\n- Jitter factor\n- Retryable status codes\n\n## API proposal\n\npython\nfrom acme_sdk.utils.retry import RetryConfig\n\nclient = AcmeClient(\n api_key="...",\n retry_config=RetryConfig(\n max_retries=5,\n base_delay=1.0,\n max_delay=60.0,\n ),\n)\n