WIP: Add V3 Objects and Capabilities

[aglensmith]

What?

• Moves all v2 resource modules to a new v2 directory
• Adds modules and classes for v3 resources to a new v3 directory
• Adds a ApiSubSubResource base class to handle requests to double-nested objects like Product Modifier Option Values
• modifies BigcommerceApi to not require client_id on init
• Adds optional version param to BigcommerceApi that determines whether to use v2 or v3 resources (defaults to v2)
• Adds new TryLatestApiResourceWrapper that’s used when BigcommerceApi version is set to ’latest' -- it will attempt to use the v3 resource, and if not found, falls back to the v2 resource.
• Makes ID optional for *.get(), and when no ID is specified, it returns same results as *.all()

To Do

• Add subresource methods to V3 resource classes (ex: Product.configurable_fields()) -- these aren’t necessary per se, but are required to reach feature parity with the v2 resources.
• Make sure class name plurality follows conventions (ProductsModifiers vs ProductModifiers vs ProductModifier)
• Remove unused classes in api.py and unused modifications to ApiSubResource base classes in base.py (since ApiSubSubResource makes those unnecessary)
• Add more tests

This is in-service of a personal project to create a bigcommerce API CLI tool written in python, but it might be beneficial to merge into bigcommerce/bigcommerce-api-python once complete and thoroughly tested.