CBG-4207: Attachment metadata migration on import

CBG-4207

Added new function MigrateAttachmentMetadata that will move any defined attachments from sync data into the new global sync xattr and remove it from sync data
Two new tests that test on demand import moving the attachment metadata and the import feed moving it
Test for import feed tests a doc that needs importing moving the metadata and a doc that is deemed as not needing to be imported migrating metadata
Define a new function for test purposes that moved global sync data attachment metadata to sync data attachment metadata

Pre-review checklist

[ ] Removed debug logging (fmt.Print, log.Print, ...)
[ ] Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
[ ] Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api