Inspector doesn't support `UnknownSchema` in metadata

[timlehr]

At this time Raven doesn't display underlying key / values of UnknownSchema objects in an items metadata. The tree can't be expanded further than the schema root.

To implement this feature, we need a new C++ API to retrieve the data dictionary from the object. See https://github.com/AcademySoftwareFoundation/OpenTimelineIO/issues/1234

Screenshot (redacted)