Move away from JSON as metadata interchange between C++ and Python

[scotts]

All of the metadata we expose to users in Python comes from the C++ side. These are mostly fields in the C++ VideoDecoder. Right now, we communicate that data from C++ to Python through a JSON string. We should explore other mechanisms so that we don't rely on JSON on the Python side.

[NicolasHug]

This might be a duplicate of #97?

[scotts]

Definitely is.