Feeds 2021-06-30 Schema Changes Not Implemented

[david-asselstine]

The base path was changed for Feeds to v2021-06-30 from v2020-09-04 but that was the only change that was made. There were changes that were made to the responses from Amazon that are not implemented and make feeds not usable.

The CreateFeedDocumentResult no longer has EncryptionDetails as a parameter.
https://github.com/amzapi/selling-partner-api-sdk/blob/master/feeds/types.gen.go#L35

The CreateFeedResponse no longer has a result of errors and now is just the values of Feeds and nextToken.

Essentially everything that had Payload as a response is no longer there.