Improvements to retry middleware

[cscatolini]

This PR adds some improvements to the retry middleware implemented.

1. Allow configuring retry exponential backoff by using RetryOptions struct in EnqueueOptions
2. Add RetryMax field: even though Retry can be either a bool or an int (when processing the job), the EnqueueOptions struct specifies it as a bool, so the maximum number of retries is always set to the default value of 25. RetryMax field allows specifying a custom maximum number of retries.

[andrehp]

Any updates on merging this improvement?

[hcliff]

any updates on this?

[fcarrero]

any update about it ?