Introduce RetryBackoffType.ExponentialWithJitter retry delay

[martintmk]

In the #1093 we introduce a new RetryBackoffType enum member. We should add a RetryBackoffType.ExponentialWithJitter enum member to it, update RetryHelper class with new delay generation and make the ExponentialWithJitter the default one.

We should consider using the code from Polly.Contrib.WaitAndRetry project for this.

See #530 for more details about the benefits of jitter delay.