Resolves #503

Summary

This PR adds support for retryCondition option for $fetch.

What’s new

• Introduces retryCondition(context) callback for custom retry logic.
• Keeps retryStatusCodes and retryCondition independent — retries if either is satisfied.
• If retryCondition throws, the error is propagated to the consumer.
• Functionality is verified via unit tests

Example

await $fetch('/api/data', {
  retry: 3,
  retryCondition: (ctx) => {
    // Retry until response body contains expected field
    return ctx.response?._data?.status !== 'ready';
  }
});