feat(utils): add retry middleware with exponential backoff

[shivv23]

Summary

Add retry logic with exponential backoff for API calls.

Why

Cloud APIs can fail due to network issues or rate limits. Retry logic improves reliability.

What

• retry() function for automatic retries
• Exponential backoff with configurable delay
• Respects CloudError retryable flag

Usage

use rustcloud::utils::retry::{retry, RetryConfig};

let result = retry(RetryConfig::new(3), || async {
    client.request().await
}).await?;

Files

• rustcloud/src/utils/retry.rs