UnknownSchema objects should allow dictionary-like access to data fields

[reinecke]

Feature Request

To allow developers to access data on objects of UnknownSchema, a dictionary-type interface should be exposed for the data fields.

Context

When the deserializer doesn't recognize the schema of an object, it deserializes to an instance of UnknownSchema. Unfortunately, the UnknownSchema object only exposes original_schema_name and original_schema_version - this means the majority data in the object is opaque and inaccessible.

See #1230 for more discussion on this.

[jminor]

I hit this again today, where I'm working with OTIO metadata which contains custom schema objects, but my code is running in a context where the proper schema plugin is not available.

I've found this kind of gross workaround for now:

if isinstance(my_metadata, otio._otio.UnknownSchema):
    my_metadata = json.loads(my_metadata.to_json_string())

It would be really nice if UnknownSchema had a .to_dict() method and/or allowed dictionary-like access via .get(key) etc.

[natchar]

I can take a look into this for Dev Days.
Cheers!