Issue #, if available:

Fixes #1758

Description of changes:

When the DefaultRateLimiter is waiting for token bucket capacity via getSendToken, the internal setTimeout delay cannot be cancelled. In environments like AWS Lambda, this means you can't abort a retry wait as the function timeout approaches — the Lambda just fails with a "task timed out" error instead of letting you handle it gracefully.

getSendToken now accepts an optional AbortSignal. When provided, the signal is raced against the delay timer:

• If the signal is already aborted, the promise rejects immediately with signal.reason.
• If the signal fires during the wait, the timer is cleared and the promise rejects with signal.reason.
• If no signal is provided, behavior is unchanged (fully backward compatible).

const rateLimiter = new DefaultRateLimiter();

// Without signal — existing behavior, no change
await rateLimiter.getSendToken();

// With signal — can now abort the wait
const controller = new AbortController();
setTimeout(() => controller.abort(new Error("Lambda timeout approaching")), 5000);
await rateLimiter.getSendToken(controller.signal);

The RateLimiter interface is updated in both @smithy/util-retry and @smithy/middleware-retry to accept the optional parameter.

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