JSON loader

[darribas]

We need a method to load a JSON file with providers and return either a new Bunch or append the JSON content to an existing Bunch. Behaviour could be:

import xyzservices.providers as xyz
import xyzservices

# Either
new_providers = xyzservices.load_json("my_list.json")

# Or:
xyz.load_json("my_list.json")

This should support both local and remote JSON files.