Add `retryAttempt` to `FetchContext`

[MrBartusek]

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?