Add support for Layout Parser

[holtskinner]

From Nithin Reddy Cheruku:

Is Document AI Toolbox compatible with the response from Layout Parser processor? We are running into the limitation of needing to traverse a nested JSON of unknown depth recursively even with the Document AI Toolbox. The documentation we have referenced: https://cloud.google.com/document-ai/docs/toolbox#use_the_client_library.