Generate "JSON encoding" functions for python types

[K-Phoen]

This PR generates a "to_json()" function for Python types, as well as a cog-specific JSON encoder that together will allow us to encode these types to JSON.

The encoder shipped with cog uses this to_json() function when it's available, or defaults to the standard encoder.

Intended usage:

from cog.encoder import JSONEncoder

dashboard = magical_function_to_generate_a_dashboard().build()
encoder = JSONEncoder(sort_keys=True, indent=2)

print(encoder.encode(dashboard))