Remove APIObject

[beckjake]

Describe the feature

I want to remove the last user of APIObject (BaseRelation and its subclasses) before we release Louisa-May-Alcott, so we can have a clean break from APIObject to hologram.JsonSchemaMixin in this release.

Describe alternatives you've considered

Leave it in the code forever?

Additional context

I've ported everything else to use hologram.JsonSchemaMixin, but Relations are still using the old APIObject directly. BaseRelation itself is a complicated beast, and there are a lot of internal and external users of the api.Relation object out there and we want to preserve compatibility with them.

Who will this benefit?

• Plugin developers, who won't need to learn both JSONSchema and hologram.
• dbt developers, who won't need to think about manually creating json schemas anymore.