[13.x] Allow passing a Closure to ThrottlesExceptions middleware

[cosmastech]

I noticed a clanker put up some code today like this:

public function middleware(): array
{
    return [
        (new ThrottlesExceptions(maxAttempts: 10, decaySeconds: 300))
            ->byJob()
            ->when(
                fn (Throwable $throwable): bool => $throwable instanceof ExternalAPIException 
                    && $throwable->isRateLimited()
            )
            ->backoff(2),
    ];
}

The ExternalAPIException has a property for reading the Retry-After header, and it would be great to be able to employ that, like so:

    return [
        (new ThrottlesExceptions(maxAttempts: 10, decaySeconds: 300))
            ->byJob()
            ->when(
                fn (Throwable $throwable): bool => $throwable instanceof ExternalAPIException 
                    && $throwable->isRateLimited()
            )
            ->backoff(fn (Throwable $throwable) => $throwable instanceof ExternalAPIException 
                ? $throwable->retryAfter() / 60
                : 2
            ),
    ];