Support remote data coming in as array or JSON-encoded string

Problem/Motivation

Some APIs or PHP SDKs return entity data as arrays or JSON-encoded strings. Support can be added to the import manager serialization so that the client adapter does not have to convert them to objects.

Remaining tasks

The MR currently works for simple list imports. We need a bit more review and testing to make sure that there are no event subscribers that require the entity before it is serialized, and to test individual entity imports as well.