Expose schema map for object serialization methods

[jchen9]

It would be convenient to expose CORE_VERSION_MAP as a Swift-friendly data type for object serialization methods like
SerializableObject's public func toJSON(url: URL, target_family_label_spec: schema_version_map = [:], indent: Int = 4) throws.

This would also be useful for writing out OTIO files in older/downgraded schema.