Remove APIObject (#1762)

[beckjake]

Convert Relation types to use hologram.JsonSchemaMixin instead of APIObject, and remove it.

Fixes #1762

• Remove APIObject
• Split relations and columns into separate files (drew was right, I should have done this initially)
• split context.common into base + common
  • base is all that's required for the config renderer
• Fixed a lot of annoying mypy things
• factored _extra behavior out of node configs
• Moved Credentials into connection contracts since that's what they really are
• Removed model_name/table_name -> consolidated to identifier
  • I hope I did not break seeds, which claimed to care about render(False) but look like they're fine
• Unified shared 'external' relation type with bigquery's own
• added hack workarounds for some import cycles with plugin registration and config p
  arsing
• Assorted backwards compatibility fixes around Relation and macros

[drewbanin]

There's a lot going on here! It looks to me like table_name can no longer be provided when creating a Relation object. That's fine, I'm glad to move on from that, but we should note it as a breaking change in the changelog!

When giving this a spin locally, I ran into an error indicating that typing_extensions wasn't installed. I was able to work around it by installing this package. I'm using python 3.7.4 -- is that expected?

LGTM pending the questions above

[beckjake]

When giving this a spin locally, I ran into an error indicating that typing_extensions wasn't installed. I was able to work around it by installing this package. I'm using python 3.7.4 -- is that expected?

I guess that should be expected, though it's really weird that the tests passed!

[drewbanin]

ship it