Add `retryAttempt` to `FetchContext`

Describe the feature

PR #372 added the ability to pass a callback to retryDelay, which should enable exponential backoff, but currently it's tricky to implement. I couldn't find an option to get the current attempt in this callback. The mentioned PR calls for context.options.retryAttempt, which doesn't exist.

I am currently using a workaround:

function exponentialBackoff(baseRetries: number, baseDelay = 1_000) {
    return (context: FetchContext<any, 'json'>) => {
      const retryLeft = context.options.retry || 0
      const currentRetry = baseRetries - retryLeft
      return baseDelay * 2 ** currentRetry
    }
  }

Additional information

• Would you be willing to help implement this feature?

Start by tracing FetchContext and the retryDelay callback behavior described in PR #372, then locate where retry options are assembled. Add the current retry attempt to context.options so exponential backoff callbacks can use it, and verify that retryDelay receives the expected value across retries.