Backoff wait sleep should check that it wouldn't exceed the context Deadline

For example, if the backoff is such that the next attempt is in 10 minutes, and the context's deadline is in 5 minutes, then the code will wait for 5 minutes for no reason.

Refer to this code:

https://github.com/hashicorp/go-retryablehttp/blob/867c4f597d1322e1642f39fd366f9df7a5c2cdc2/client.go#L771-L791

Before the `select` block, the future retry time should be checked to see if it is beyond the current context's deadline, and return early error in that case.

Also side note: the use of `time.Timer` and `.Stop()` is unnecessary and can just be `time.After(...)` in modern go, and even if backwards compatibility is an issue then `defer timer.Stop()` should be preferred...

So, how about this?
```
nextTime := time.Now().Add(wait)
if cd, ok := ctx.Deadline(); ok && nextTime.After(cd) {
return context.DeadlineExceeded // Its the same error we would have returned, we're just returning it earlier...
}
select {
case <-req.Context().Done():
c.HTTPClient.CloseIdleConnections()
return nil, req.Context().Err()
case <-time.After(time.Until(nextTime)):
}
```