Add support for snapshot/hourly feed files

[zcourts]

It took me some time to work out why I couldn't find a way to download hourly update/snapshot feeds according to https://developer.ebay.com/api-docs/buy/feed/resources/item_snapshot/methods/getItemSnapshotFeed

This lib already does all the hard work for piecing the partial downloads together etc but skipped out on snapshot support.

I hacked it by using reflection to replace the feedUtils field with an object that overrides String getFinalUrl(FeedRequest feedRequest) but ideally this would be supported.