Measure latency/request duration and provide it for logging purpose

Right now it's hardly possible to get an attempt latency for a log like `"request failed after 300ms, attempts: 3"`.
It's quite important to know how much time was actually spent per each request, since the external service can abort it at any moment. And in case the backoff didn't help -- to know the total duration in the [default error](https://github.com/hashicorp/go-retryablehttp/blob/40b0cad1633fd521cee5884724fcf03d039aaf3f/client.go#L824) within the number of attempts.

Ideally, `ResponseLogHook` could be extended with more arguments like `attemptsNumber`, `attemptLatency` (last one), `totalLatency` (sum of all attempts + delays)
All of it can be structured under some "Details" structure.

For `ErrorHandler` it could also be useful to have the final total latency.