Add AbortController Integration

[AryanSharma48]

Modern applications frequently use AbortController to cancel requests.

SmoothAPI should support and correctly propagate external abort signals throughout the retry lifecycle.

Example

const controller = new AbortController();

fetchWithRetry(url, {
  signal: controller.signal
});

Acceptance Criteria

-Support external AbortController signals
-Stop retries when request is aborted
-Add tests
-Update documentation
-Add example usage