Add support for bulk creation of objects with multi-value attributes

It appears that we may have completely overlooked the ability to create objects in bulk that have multi-value attributes (e.g. where the attribute on the server side has `multi=True`).

We'll need to come up with a way to represent these. It might be better to reconsider the entire format of the bulk file to something more descriptive than the "colon-separated" format we have now.

Start by tracing the current colon-separated bulk-file handling and the object-creation path in the Python client and CLI. Determine how multi-value attributes are represented and define the bulk format changes needed; done means bulk creation accepts objects whose server-side attributes use multi=True.

Written by the indexing model from the issue text.