Implement canDownloadFiles and canManageFilesPermissions fields for dataset user permission  #217

Inspired from this issue/TODO in front-end repo, in Dataset Permissions, it asked about adding canDownloadFiles and canManageFilesPermissions permission fields to js-dataverse. Based on the dataset user permission model in js-dataverse repo, we don't have these two fields yet.

In native API, one endpoint Know If a User Can Download at Least One File from a Dataset Version may be relevant to the canDownloadFiles field.