TUN-3902: Add jitter to backoffhandler

Jitter is important to avoid every cloudflared in the world trying to
reconnect at t=1, 2, 4, etc. That could overwhelm the backend. But
if each cloudflared randomly waits for up to 2, then up to 4, then up
to 8 etc, then the retries get spread out evenly across time.

On average, wait times should be the same (e.g. instead of waiting for
exactly 1 second, cloudflared will wait betweeen 0 and 2 seconds).

This is the "Full Jitter" algorithm from https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/

origin/reconnect.go

@@ -108,7 +108,7 @@ func (cm *reconnectCredentialManager) RefreshAuth(
 	authOutcome, err := authenticate(ctx, backoff.Retries())
 	if err != nil {
 		cm.authFail.WithLabelValues(err.Error()).Inc()
-		if _, ok := backoff.GetBackoffDuration(ctx); ok {
+		if _, ok := backoff.GetMaxBackoffDuration(ctx); ok {
 			return backoff.BackoffTimer(), nil
 		}
 		return nil, err