adding resilience4j-retry to integration tests

[ronreynolds]

i keep seeing ITs failing with "com.smartsheet.api.ResourceNotFoundException: Not Found" after the creation of something (typically a Sheet or Folder) and the immediately subsequent attempt to use the ID returned from the server. i suspect this is a lag between creation of a resource by 1 server and its subsequent visibility to other servers/services. in the case of app-core it would normally trigger a cache miss and subsequent read from the DB but it's possible it's not a cache miss as much as replication delay between the writable primary grid-shard and the read-only replica grid-shards.

the cleanest way i can think of at the moment is to add resilience4j-retry around those subsequent calls to add retry with backoff. resilience4j is the successor of Netflix-Hystrix and, last i checked, is the best-in-breed solution to this sort of problem. details about it are here - https://resilience4j.readme.io/docs/retry and i'm going to add it to one of my branches that are currently failing due in the IT stage in the above way.

i had considered adding a simple Resilience util class with a retry() method taking a lambda to the Smartsheet Callable and a few settings but this wouldn't be something SDK users could grab off the shelf (as we don't currently have a non-internal (i.e., public) util package in the SDK and Res4J is maintained and it'll work with the OpenAPI version and it's almost certainly better in almost every way than whatever i would write in a pinch.

opinions very much welcome. :)