Dataset Size API

[djbrooke]

We have an endpoint for getting Dataverse size, added in #5848/#5941:

http://guides.dataverse.org/en/latest/api/native-api.html#report-the-data-file-size-of-a-dataverse

We should add an additional endpoint for getting dataset size that should (generally) follow the same logic as the dataverse API. The display of this on the dataset page (#6400) is still being designed and tested, but we should add the API now to ease UI development in the future and because this may be useful for systems integrating with Dataverse.

[djbrooke]

Three use cases:

• I'm an administrator and I want to know the size of a dataset added by a researcher so that I can take administrative action
• I'm a user and I want to know how much data will be included in the download if I try to zip all files and download
• I'm an integrating system (Whole Tale, etc) and I want to provide some expectation in my UI about how long something will take when pulling files from Dataverse

Notes:

• We should provide a mode for what will be displayed on the page (and included in the zip) and also the storage being used (for administrative purposes)
• We should be able to use code from 5848 api dataverse totalsize #5941