30441 Add tests for replicating attachments to ReplicationAcceptance

This PR has these major contributions:

* Add a good set of tests for the interoperability between the new API for attachments and TouchDB's replication code.
* Fix a bug where attachments saved to disk in gzip format by TouchDB's code wouldn't be unzipped by the new API.
* Switches to using NSData rather than NSInputStream as the basic data type, as input streams seemed like they would be a pain to deal with for objc developers (they generally live quite far down the abstraction stack, unlike java).
* Fixes a bug that meant that the CDTDocumentRevision objects returned from -getAllDocuments couldn't be used to get attachments.